AR-15 Rate of Fire as a Function of Buffer Weight and Action Spring – Initial Comparison

I must stress that this is an initial comparison based on limited data points, but I feel that it will be fairly accurate once more data has been gathered.

Many rounds were fired through a number of uppers, but I spent the most time with a 20″ rifle gas upper; data can be seen below. The barrel used had a 5.56mm chamber and a .092″ gas port, which are standard M16A2/A4 features.

Although the BCM action spring and the “generic” action spring were identical in appearance, their performance was quite different. Given the same weapon, recoil buffer, magazine, ammunition, environmental conditions, and even number of rounds in the magazine, the BCM spring reduced rate of fire by approximately 70rpm compared to the generic spring.

Time in each component of bolt carrier travel was reduced, however, the most significant change occurred during the “hang time” where the bolt carrier group was at its rearmost point of travel; the BCM spring delayed forward movement by as much as 35%. In terms of real world performance, the BCM spring allowed 35% more time for the magazine spring to properly feed the “stack” of ammunition.

Even the Wolff reduced power spring slowed rate of fire more than the generic spring. All springs had less than 300 rounds/cycles “on” them prior to testing.

While the switch from Carbine to H buffers resulted in a significant (~50rpm) drop, going from H to H3 – double the weight difference of the carbine to the H – only reduced rate of fire by 6-13rpm. I have many theories, only some of which are grounded in reality. It may be that simply having any amount of tungsten in the buffer changes the way the buffer acts when it reaches the rearmost point in the receiver extension tube, for the average rearward velocities of the H and H3 buffers were nearly identical, whereas other times and velocities differed.

Although I am not ready to release data for other uppers (even on an “initial” basis), their behavior with the above buffers and springs seemed similar.

Before anyone asks, I don’t have sufficient data for the H3/generic spring yet.

Rates of fire were calculated on high speed video, which was calibrated prior to testing. High speed video of my uppers on registered, full auto lowers were used to calculate theoretical rates of fire for semi-auto lowers. Rate of fire reduction is not the only reason to select an action spring and buffer. Your individual results may vary based on dozens of factors.

Full Auto Fun – Er, Testing

Two quiet, unassuming gentlemen arrived at the range with some serious hardware while I was shooting. Naturally, I pestered them until they let me shoot one of my uppers on a Colt M16 lower.

I was able to take a fair amount of high speed video, which I will be processing a bit at a time…here are two short clips…

1911 Drop Testing

Although this is the sort of thing that I do occasionally, and have in fact conducted in the past, it is important to note that I had nothing to do with the testing shown below, and am simply reposting it from another location (with permission from the owner of the data). Drake is a law enforcement officer and 1911 gunsmith, and has done some excellent testing here.

1911 Drop Testing
The original testing used a 9mm steel firing pin and a 9mm titanium firing pin. The firing pin hole was then reamed for a .45 sized pin and the tests were repeated with .45 sized steel and titanium firing pins. All of the firing pins were weighed prior to testing. A Wolff XP firing pin return spring was used for all of the testing. All of the cases used for testing used Winchester large pistol primers. The frame and slide were donated by Gary Smith at Caspian. The pistol was built using techniques learned from Larry Vickers and Bruce Gray. The pistol was tied to a section of 550 cord, looped over a pulley, and dropped onto common floor materials. The magazine was loaded with 8 dummy rounds to bring the pistol up to proper weight. Four floor types were selected. Concrete, Pergo, 5/8 plywood, and shag type carpeting. The thumb safety was left OFF as preliminary testing with the safety ON indicated that damage to the thumb safety, slide, and plunger tube would occur with only a few drops. The hammer frequently dropped to the half cock notch during testing.
Firing Pin Weights:
9mm STI titanium pin— 2.17 grams
9mm Caspian steel pin — 4.45 grams
.45 STI titanium pin — 2.36 grams
.45 Colt steel pin — 4.30 grams
I was amazed at how easily a Series 70 1911 could be drop fired. Steel firing pins and concrete are a bad combination. 9mm sized pins and titanium construction will add several feet to your safe drop distance. I will be running Wolff XP springs and a Ti pin in all of my Series 70 type 1911’s.
I have attached an Excel spread sheet with the results. You will notice a lot of “Did Not Drop” entries. I saw no reason to drop test a particular combination of firing pin and flooring if it was not firing at higher distances or on harder flooring. I did several drops at various distances to get an idea of safe drop distances. This was to account for hard or sensitive primers. Each primed case was dropped only once. Just in case you were wondering, the pistol sustained significant damage. The muzzle is distorted from being dropped. I had to turn down the outside diameter of the barrel three times just to keep the slide from locking up. The muzzle, magwell, and grip safety have some serious blending in their future. Nothing sounds worse than a 1911 hitting the concrete from 10 feet!

The Excel file is attached here.

Does Nickel Boron Reduce Heat?

We’ve been bombarded with a variety of coatings and platings over the past few years, most of which are called “proprietary” and given a cool name. In reality, there aren’t a whole lot of finishes or metal treatments out there. Many are just variations on a theme, such as all the derivatives of nitrocarburization and nickel plating.

Nickel boron is related to electroless nickel plating and electroless nickel with teflon (also known as Robar’s NP3) with regard to the plating process. Nickel boron is reported to provide “permanent dry lubricity”. In other words, less friction amongst the reciprocating parts, as well as where they interface with static components.

It didn’t really occur to me that this might lead to lower operating temperatures. In fact, when I noticed a discrepancy between a rifle with a nickel boron BCG and a similar rifle with a standard, phosphate finished BCG, I wasn’t sure what to think. They used different rail systems and gas system lengths. Initially, I chalked it up to those minor differences. Then, I decided to use a standard BCG in the rifle that was originally equipped with the NiB BCG, using the same test protocol. After that, I repeated the test, twice, with each BCG, allowing the weapon to cool to ambient temperature between strings of fire.

The test was performed by firing 80 rounds of centerfire ammunition as quickly as possible through the AR-15 pictured below:

No malfunctions were experienced during any of the 5 strings of fire.

I then measured the temperature of the gas block, chamber, bolt face, and handguard (in four locations) immediately after firing and at two minute intervals out to 10 minutes.

For the sake of comparison, I have included the temperature of an M4 type carbine equipped with a KAC M4 RAS handguard. The rifle above was equipped with a Daniel Defense OmegaX 9.0. None of the rails had any covers during the testing.

Gas block temperature profiles were nearly identical for all weapons.

The same goes for chamber temperatures.

Bolt face temperatures, however, were another story.

The bolt face of the nickel boron plated BCG stayed, at its peak, 13 degrees cooler than the same weapon with the standard BCG, and 17 degrees cooler than the M4 carbine with the KAC M4 RAS.

Here is the nickel boron BCG compared with the POF RDIK and POF P-415 uppers, which underwent the same test (again, we’re talking bolt face temp here):

As always, I’m not a scientist and this was not a scientific test, but I do feel fairly confident in the results, given that I double and triple-checked the numbers and nothing was outside of a small margin of error. The above numbers are the average of said tests and retests.

I do realize that this was a sample size of one and that the limited testing doesn’t definitively prove anything. I do think that it is an interesting result that I would like to follow up on after I get more ammo and possibly more nickel boron plated BCGs.

Heat Dissipation: Insulate or Circulate? Gas Tube or Op-Rod?

Standard disclaimer: I’m not a scientist and this was not a scientific test. Any conjecture on my part is purely an uneducated guess.

As I’ve written before, POF-USA provided me with two of their upper receivers – one is of their standard P-415 design and the other is actually operated via a standard gas tube. It’s called the RDIK.

This gave me the opportunity to compare how each handled heat. That is, just how effective are all the design changes POF has made to the AR receiver, barrel nut, and handguard? Well, as I found out, they’re quite effective. However, that test was pretty limited – only 30 rounds per weapon – and I wanted to step it up a little.

Today I put 80 rounds through each of three ARs – the P-415, the POF RDIK, and an M4 type AR with double heat shield handguards – and will shoot more in the next few days with other weapons. I also took chamber and bolt face temperature readings, in addition to the handguard temp (average of 4 places on the handguards) and gas block/barrel temp.

The rounds were fired as quickly as possible, and the rifles were left with the bolt carrier group in the forward and locked position. Temperature readings were taken immediately after firing and at two minute intervals thereafter, out to 12 minutes post fire.

We’ll start with handguard temperature.

As you can see, the M4’s double heat shield handguards were much hotter than either POF offering. The POF RDIK, in fact, had a slightly cooler handguard than the POF P-415.

This was in part due to the very hot gas block of the P-415. Here are those temperatures:

It wasn’t quite as scorching as the M4’s 353 degrees immediately after shooting, but it was over 320. The POF RDIK was drastically cooler – it never exceeded 200 degrees.

Chamber temperatures were much closer for all weapons.

The P-415 did stay cooler than the RDIK, with a difference of  roughly 10 degrees. The M4 was hotter than either of the POF weapons, due in no small part to the heat sink barrel nut used on the POF rifles.

The following graph shows bolt face temperature.

It would appear that a large portion of the heat reaching the standard AR-15’s bolt comes from the front – that is, the chamber. If we compare chamber and bolt temperatures, the RDIK and M4 hardly ever had more than a 2 degree difference between the chamber and the bolt (with the bolt normally being 1-2 degrees cooler than the chamber). The P-415 bolt, on the other hand, generally stayed about 10 degrees cooler than the chamber.

What does this all mean? Well, to me, it means that getting the heat out (circulating air) is more important than trying to keep the handguards cooler (insulating the barrel with double heat shields) – regardless of the operating system you choose. It would appear that the piston/op-rod P-415 does slightly reduce bolt face temperature – but the RDIK does a very fine job of keeping the chamber area cool in its own right, which in turn keeps the bolt cooler.

It seems that there is no free lunch, and the heat which is not present in the P-415 chamber and bolt is very present at the gas block. The heat sink features and wide open handguard with lots of cooling slots almost seem necessary to keep the barrel/gas block temperature relatively in line with that of the M4 type AR. I would really like to test an op-rod conversion that does not have the heat sink barrel nut, big handguard, etc.

I would assume, based on these graphs and the comparison of the three uppers, that the large majority of the temperature of an AR-15 bolt during sustained fire can be attributed to the “fire in the barrel”, and a minority comes from the gas which circulates through the action. In other words, with the piston/op-rod system, the chamber “heats” the bolt, whereas in the standard operating system, the bolt is heated not only by the chamber but in a small way by the gas coming through the gas key, which in turn causes the bolt to pass some heat back to the chamber. As a result, the temperature of the bolt and chamber on a standard AR are married to one another to a greater degree (ha, ha) than on the P-415.

Again, I’m not a scientist. If anyone has a better conclusion based on the above data, I’m all ears.

Federal XM9HA Chronograph Test

In a recent AR15.com thread, it was claimed that Federal 9mm ammunition identified as XM9HA was a contract overrun of 147gr HST hollow point projectiles loaded to the impressive velocity of 1180 feet per second. That’s 9mm Major territory – and beyond. It was also reported that this ammunition had a high rate of failures to feed or failures to fire in a variety of handguns.

I was recently sent a small quantity of this ammunition for testing via a chronograph. It was requested that I use a Glock 19 as the test firearm.

Although I only fired 10 rounds through said Glock 19, I did not experience any failures to feed or fire. Unfortunately, I also did not experience anything approaching the claimed velocity. The fastest round was 1009.42fps, the slowest 973.06fps – with an average of 992.56fps.

For comparison, I also fired 10 rounds of Winchester Ranger Bonded 147gr through the same firearm. The high was 992.21, the low 963.15, with an average of 974.8.

If you can acquire this XM9HA ammunition for a fair price, and it functions reliably in your firearm, it would appear to be adequate ammunition. However, I wouldn’t expect 9x23mm performance from this cartridge. As a side note, recoil felt pretty tame compared to my 115gr handloads at 1200fps (which are not particularly hot to begin with).

LWRC IAR Drop Test

I’ve been told that the following information is OK to release. It’s pretty self explanatory. I removed the names of certain individuals, but the information is otherwise unedited.

Land Warfare Resources Corporation

Engineering Memorandum Engineering Memorandum

To: IAR Team

From:

Date Written: 8/8/07

IAR Drop Test – Summary

Purpose: The internal drop testing of the LWRC Infantry Automatic Rifle

was undertaken to determine how robust the Rifle and its component parts

and accessories are and simulates a drop from a rotary winged aircraft at

the maximum height a soldier might disembark from a rotary winged

aircraft or from the top of an Armored Personnel Carrier or Tank. The

purposed was also to find the best commercial off the shelf parts and

accessories that meet the requirements of the USMC IAR Draft RFP.

Testing Protocol: The LWRC Infantry Automatic Rifle was drop tested to

the USMC test protocol to ensure durability of the weapon and integral

accessories and furniture for internal product improvement prior to delivery

of LRIPS for Phase 2 of the Picatinny IAR Solicitaion. The weapon as

tested was 8.5 lbs. The weapon was dropped several times from all

angles starting with 12 o’clock butt down from 1.7 Meters (from the

buttplate) with the stock extended and locked in the 3rd position on the

buffer tube. The surface it was dropped on was concrete covered in 3/8”

plywood. For the purposes of the test, no optics were installed on the

weapon. The first drop being 12 o’clock butt down would test the viability

of the telescoping stock systems in the extended position. The testing

continued dropping the rifle from the 2, 3, 5, 6, 7, 8, 9, 10, 11 o’clock

positions from 1.7 m on the same surface. Back up rear Iron Sights were

in the upright position.

Pass/Fail: Success or failure of any component or accessory was judged

by whether the component was still operable after the drop testing. The

component could be damaged, but the weapon had to remain operable

and the repair of the replacement part must have been within the scope of

practice of a line infantry armorer.

Components Tested: Magpul CTR stock, Magpul MIAD, LMT SOPMOD

stock, A2 Gov Issue Pistol Grip, Latest Generation M4 Stock, VLTOR

EMOD, IAR Prototype Config, Matech Rear BUIS, TROY BUIS (sights are

purely educational as USMC has specified rear sight).

Failures Summary: All buttstocks failed the testing first drop except for

the VLTOR EMOD stock. The VLTOR EMOD stock survived all drops

from all angles and remained completely functional. Examination after the

testing revealed 2 hairline cracks from the corners of the steel locking plate

hole in the polymer. The stock remains completely intact and functional

and the engineering team has determined the stock would not need to be

replaced to serve the life span of the weapon. The Matech Rear sight

body cracked but remained functional. The Troy Sight broke during angle

drops and was rendered non functional. All other IAR Prototype

Components, and the weapon itself were inspected gauged and

measured, then test fired. The weapon underwent 48 drops (far beyond

the requirement) without any functional problems. There were no pistol

grip failures.

Land Warfare Research Corporation

Heat Dissipation: Two Schools of Thought

Heat is discussed fairly often on various internet forums, especially when two subjects come up: barrel profile and method of operation.

We often see comments about how light barrels heat up too quickly. This is partially true – a lighter barrel will generally heat up faster than a heavy barrel. The “too” part is where the problem lies. Too fast for a machine gun barrel? Most likely. When you’re putting out a sustained rate of fire that can reach several hundred rounds per minute, a light barrel is definitely unsatisfactory. However, if you have a rifle, and not a machine gun, a lighter profile barrel may not heat up “too quickly”.

Also, there are many comments about how cool piston/op-rod systems run. These comments seem to be applied liberally and generally; that is, you will often hear that all piston conversions “run way cooler” than standard DI weapons. However, it’s not as if the mere presence of the op-rod has a chilling effect on the barrel, which is a critical component of the rifle, to be sure.

So we have two schools of thought here: that a lightweight barrel profile is more appropriate for use on a carbine, and that the standard system of operation is not unnecessarily hot; and that a heavy (or fluted/heavy) barrel is more appropriate for use on a carbine (or maybe a carbine machine gun to be used for laying down suppressive fire), and that an op-rod allows the rifle to run cooler.

Recently, while doing some experiments with standard plastic handguards, I thought I’d also compare a civilian legal M4 clone, or close to it – a Spike’s Tactical M4 LE with a Knight’s Armament M4 RAS handguard – with two Patriot Ordnance Factory rifles. One is a P-415, which uses POF’s op-rod system, and the other is called the RDIK, and it uses a gas tube, just like a standard AR-15. However, it’s equipped with the same heavy fluted barrel, heat sink gas block, reinforced upper receiver, and single piece railed forend that the P-415 uses.

To complete this test, I fired 30 rounds of 5.56mm ammunition through each rifle, then measured the temperature of the handguards in four separate places, as well as the temperature of the gas block/barrel. I measured these temperatures immediately after firing, 1 minute after firing, 5 minutes after firing, and 10 minutes after firing, using an infrared thermometer. I also measured the temperature of the bolt face immediately after firing.

Here are the handguard temperatures:

Note that all three handguards got progressively hotter until sometime after the 5 minute mark. I found it interesting that the POF gas tube upper was within several degrees – up or down – of the P-415 op-rod upper during the whole exercise. I also found it interesting that every handguard had reached essentially the same temperature 10 minutes after firing.

Here are the gas block/barrel temperatures.

I was surprised to see how cool the POF RDIK gas block was after firing. There was a greater initial temperature difference between it and the P-415 gas block than there was between the P-415 and the Spike’s M4 LE. In addition, the M4 cooled faster than the P-415, as you can see, though each upper had roughly the same temperature loss profile after 1 minute.

This proves an often-overlooked point: while light barrels do heat up faster than heavy barrels, they also cool down faster than heavy barrels – apparently, faster than even a heavy fluted barrel. This also proves true a comment made to me by an industry professional while we were discussing this topic: that the gas block of a piston/op-rod rifle gets very, very hot.

As a side note, the temperature of the M4’s bolt immediately after firing was 94 degrees; the P-415’s bolt temperature was 88 degrees. The RDIK’s bolt was 89 degrees.

Although this was a very limited and rather unscientific test, it would seem that the vast majority of the POF rifles’ cooling ability comes from the heat sink barrel nut, handguard, fluting, etc, and not from the piston/op-rod system. I will do more extensive testing in the near future.

KAC M4 RAS Heat Dissipation

I’ve updated my earlier heat dissipation graphs with the KAC M4 RAS. This is the railed handguard used by the US military on Colt M4 carbines and similar weapons. I did not have rail covers on the handguard during this test, because I was also comparing it to some other railed handguards.

Here are the handguard temps:

And the barrel/gas block temps:

Not only did the M4 RAS transmit less heat to the shooter’s non-firing hand, it also allowed the barrel to cool faster than the plastic handguards. This is in addition to its other benefits, such as mounting various accessories.

Heat Dissipation Update: Magpul MOE

This is an update to the earlier heat dissipation comparison between the single and double heat shield handguards.

Today, I tested the Magpul MOE handguards using the same protocol: 28 rounds fired in a rapid manner, with temperature measurements at specific locations and time intervals.

The MOE handguards have a single heat shield. By that, I mean a single heat shield. The “single heat shield” handguards have one heat shield in each of two handguard halves, and the double heat shield handguards have two heat shields in each of two handguard halves. The MOE has one heat shield in the lower handguard half, and no heat shield in the upper half. This might sound bad from a “protect the user” standpoint, but it has many more vent holes than the other handguard styles, and these holes are located to draw in cool air on the bottom and expel hot air up top.

As to the effectiveness of the design, I’ll let the results speak for themselves. I was pretty impressed. First, the handguard temperature averages.

And here are the gas block/barrel temperatures.

Based on the results of this limited test, it seems that the MOE offers a very good compromise between reducing heat transmitted to the user’s non-firing hand and allowing heat to escape, resulting in slightly faster barrel cooling.