Lately it seems as if there have been a rash threads about “tuning” a particular set up. Whether it’s truly needed or it is a perception of the individual, in the end it is for that person to decide. Let’s take a look at the various factors that would lead someone to do this and provide considerations that need to be dealt with.
We have a standard in the AR world. It is the Colt M4 carbine or the Colt M16 variant rifle. Quite simply these weapons are proven and we can use them to help guide us if we understand some facts about them.
THE RIFLE
Gas System- The rifle uses a rifle gas system with a port size of .093.
Buffer Weight- The rifle buffer weighs 5.1-5.3 oz.
Spring- The rifle uses a rifle length spring and should measure from 11 3/4 to 13 1/2 inches.
Ammo- For optimal function one needs to use full powered MILSPEC ammo. This means M193, M855, MK 262 MOD1 or similar.
BCG- The bolt carrier group is a full auto M16 style.
Lower Receiver Extension (Buffer Tube)- The rifle uses a full length rifle tube.
Suppressor- Are you using or planning to use a suppressor?
THE CARBINE
A carbine is essentially a lightweight more compact version of the rifle. In addition to cosmetic differences it uses a different gas system.
Gas System- The carbine uses a carbine gas system with a port size of .063 It may also use a midlength gas system and there is no standard for a midlength port. They are usually anywhere from .076-.080. THIS IS A GENERAL GUIDELINE.
Buffer Weight- The standard carbine buffer weighs approx. 2.9 oz. However, Colt M4/M4A1 carbines use the H2 buffer and the Colt 6920 carbine uses an H buffer. In addition to the slight difference in barrel length we must realize that M4/1A series is also full auto or busrt which is why the H2 buffer is used.
Spring- The carbine uses a carbine length spring and should measure from 10 1/16 to 11 1/4 inches.
Ammo- For optimal function one needs to use full powered MILSPEC ammo. This means M193, M855, MK 262 MOD1 or similar.
BCG- The bolt carrier group is a full auto M16 style.
Lower Receiver Extension (Buffer Tube)- The carbine uses a carbine length tube usually a 4 position or 6 position.
Suppressor- Are you using or planning to use a suppressor?
So now that we have that information you need to ask yourself what is the intended goal of your “tuning” effort. Are you simply wanting to slow down the cyclic rate or are you compensating for something?
First and foremost any modification that is made should not affect the operational reliability of the weapon. That means quite simply if you add a heavier or lighter buffer and you encounter malfunctions (when it previously worked) something is wrong and you need to put it back into the original state.
If you have an overgassed weapon (i.e. gas port is too large) then adding a heavier buffer can provide a quick immediate fix to bolt bounce or similar issues. Understand that the underlying problem is the gas port.
If adding a suppressor (which also creates an overgassing effect) then adding a heavier buffer will help to smooth out the operation. In many cases it will solve issues related to the failure of the bolt lock to lock back after the last round was fired.
Ammunition is a major factor here as well. If you are shooting non 5.56 loadings you may not be able to run the heavier buffers. Ultimately whatever you choose, you should be ensuring that your weapon will function with all the ammunition that you have in your stock.
Springs can also affect the operation. This is due to material construction and spring pitch.
So now we can see that if we start to change the pieces of the puzzle we can affect the overall operation of the weapon. All of the above factors need to be considered when you make those changes.
The wild card in all this is the midlength system which due to the fact there is no standard and the variations in port sizes one should attempt to get the port information. Additionally we can also look at some companies such as BCM or Noveske to see what kind of buffers they ship out with their completed carbines. It has been my experience that most properly spec’d carbine and midlength system guns will run with an H buffer and quite frequently an H2. Whenever I have concerns I also check my stuff with the lightest or weakest load in my ammo inventory and go from there.
Might as well sticky this right away so we can point the under educated and over thinkers this way.
Oh, and also you should include some info on the ejection chart. That’s seems to get some people in a tizzy. I really like point that was made the other day about it being more important where your bullets end up than where the brass does. Some people get too caught up in this.
I find that some people over think the “tuning” aspect of their guns.
A close friend of mine, one who build and tuned my race cars and many of the fastest cars in that platform in the country built an AR a while back. He spent hours and countless rounds trying to make sure that when the rifle ejected the spent case there was no mark on the brass from the shell deflector. Not like the rifle was leaving huge dents in the brass, they were so minor you would have to run your nail over them to feel them. He went crazy with gas port sizing and buffers… meanwhile it was with only one type of ammo. I guess this is what happens when you build race engines and have access to every kind of CNC machine imaginable lol.
I once got to talk with some engineers from the Mclaren Race Team. We conversed for about 3 hours. I did most of the listening. I can’t believe what they go through to get an engine around the track for 500 miles-and believe me, the work is for 500 miles exactly-if that is what the race calls for. Back on topic. IG, since you posted “101” please follow this up with a 102, 103, etc. I (we) appreciate the info.
Indeed perhaps a how-to guide, including ranges of various known factory ammo pressures. Though, this info is already on the board if one does a little searching.
Excellent write up, IG, and much appreciated. I’m printing this and putting it in with my tools and spare parts.
I also stickied the thread. Do you want this to stay in AR General Discussion, or want me to move it to Ar Technical Discussion? Seems like a good fit in either location.
FYI, my Colt 6520 purchased new in 2010 came with an H2 buffer like their M4/M4A1 carbines, rather than the H buffer like their 6920. Has Colt switched back to the H?
Great post!
If I may add to this section about tuning, and ammo selection.
Don’t forget to consider cold weather, lubrication and fouling when tuning the rifle to your ammo.
If you tune your rifle to your least powerful loading and the rifle JUST functions when using it, you will begin to have failures when your rifle gets dirty, looses its lubrication or is used in colder weather.
You have to allow for function during these conditions or you will be dealing with a rifle that works great clean and slick but as the day progresses, you will encounter problems.
In my short experience I try and slow down the cycling rate as much as I can with buffer weight and maintain function. I am not sure if this is the right thought process or not, but it seems to work. I run an H2 in a midlength with shit Russian ammo reliably. I could not detect bolt bounce when running 5.56 (mil pressure) ammo. I assume with higher pressure ammo the gun will run faster, but if I set it up for 5.56 it may not run shit .223 well. I try to set it up for the least common denominator.
Off topic warning - I ran into 3 McLaren mechanics during the 2006 US Grand Prix in Indy… in a gun store that rented firearms/range in the back lol. Poor Brits without guns… The conversation did not go well as I am a crazed Ferrari Fan