"trimming" a barrel

[cyras21]

I was looking at a shorter barrel for my AR15 and was wondering if it could just be cut and refitted with a flash suppressor. Any one know if this can be done and possibly recommend someone?

[scrubber3]

There is a lot to consider when cutting a barrel. Doing so without taking into account quite a few variables could refer the chopped barrel useless. The twist rate and length by which it needs to achieve spin on the round. The end of the barrel must be tapered so the bullet will leave without issue as well. Also, you must consider any extra pressure that may be put on the gas block as a result of the shortened barrel.

I'd say it's best to let a trained professional do it. Or my advice would be to buy a shorter barrel. Whatever you do, just make sure that the length stays within the specs of the law. Otherwise you risk a felony.

[cyras21]

oh...there's no way I would be doing this myself. Was just wondering if it was a cheaper option.

[Jakeiscrazy]

Like Scrubber said your shorter barrels need a different twist rate unless your just taking it down buy a few inches. Even if just a few inches you still need to recrown it and for the cost of tools you could have a pro do it.

[jdonovan]

to do it right, you'd really want to remove the barrel from the upper. At that point you might as well just install a factory barrel of the appropriate length.

You can do it cheaper... but do you really want to?

[Sensai]

OK, now for the real story! First, the bullet doesn't "spin up" by the length of the barrel. The bullet takes a set and spins at the twist rate of the barrel almost immediately. If there's any, or at least very much, skidding while the bullet gets to it's spin rate you have a much bigger problem than barrel length. As far as the end of the barrel being tapered so the bullet leaves well, the end of the barrel, or "crown" must be as near perpendicular to the axis of the bore as possible so the the propelling gases don't have more pressure on one side of the bullet than the other at exit. The only reason for barrel taper is weight savings. The real culprit in shortening a barrel in a gas operated system is that you decrease the time that the pressure is allowed to operate the system before dropping to atmospheric pressure when the bullet leaves the barrel. The size of the gas port(s) and barrel length after the gas port are finely matched parameters. If one is changed, the other must be adjusted to match.

Sorry for being so long winded, but blatent misinformation kinda ticks me off!

[scrubber3]

OK, now for the real story! First, the bullet doesn't "spin up" by the length of the barrel. The bullet takes a set and spins at the twist rate of the barrel almost immediately. If there's any, or at least very much, skidding while the bullet gets to it's spin rate you have a much bigger problem than barrel length. As far as the end of the barrel being tapered so the bullet leaves well, the end of the barrel, or "crown" must be as near perpendicular to the axis of the bore as possible so the the propelling gases don't have more pressure on one side of the bullet than the other at exit. The only reason for barrel taper is weight savings. The real culprit in shortening a barrel in a gas operated system is that you decrease the time that the pressure is allowed to operate the system before dropping to atmospheric pressure when the bullet leaves the barrel. The size of the gas port(s) and barrel length after the gas port are finely matched parameters. If one is changed, the other must be adjusted to match.

Sorry for being so long winded, but blatent misinformation kinda ticks me off!

Wow.... Tapered barrel, weight saving?
Twist rate immediately?
Sorry chief, but that's misinformation. No offense though. The stuff I posted didn't come off of the Internet. It's what custom gun builders will tell you. Maybe not word for word, but round about just the same.

[Sensai]

I instructed, was chief instructor and finally manager of the Army Aviation Armament course. I've conducted more internal, external and terminal balistics tests than most people could imagine. What are your credentials?

As far as the bullet taking the twist rate almost immediately, think about it. If the bullet didn't, it would skid and the grooves forged into the bullet would be wider than the rifling in the barrel. If you recover a spent bullet you may find a slightly wider groove near the nose, but it will be the same width as the rifling further down the projectile. If it was wider the full length of the bullet there would be no gas seal because the propellant gases would flow by in the space between rifling and bullet groove. This causes gas cutting and metal fouling of the barrel. This is the primary reason you don't see many progressive rifled barrels any more. You also have to remember that the bullet is entering the rifling and having the grooves swaged into it at the slowest speed and highest pressure it encounters in it's progress down the barrel.

The exterior taper of the barrel is only for weight savings. The reason for a heavy or "bull" barrel has nothing to do with accuracy, it's to keep the barrel cooler longer with repeated firing. The internal ballistics is the same if you bore and rifle a skinny barrel profile or a ten inch block of steel. The only accuracy consideration deals with vibration and harmonics. If you're talking about interior taper, there's no such thing. The only interior taper is between the mouth of the chamber and the start of the rifling. This is the throat and is (can be) an enfluence on how well the bullet takes the rifling. Some manufacturers prefer a short throat and others prefer a long or "freebore" throat. As with most things, there are advantages and disadvantages to both. At any rate the throat will not be a factor in shortening a barrel.

The crown of a barrel, the very end of the bore, is very important to accuracy, but not so much to the operation of the action. Basically the cleaner the departing projectile is released from the barrel the better for accuracy. The reason for some barrels having recessed crowns is to protect the crown from physical damage. It is a compromise, the absolute best crown is one that is perfectly perpendicular to the bore axis and even on all sides all the way around.

By the way, as far as what custom gun builders say, I am one and I don't spread that garbage.

[scrubber3]

So, if you took a 12:1 twist barrel and chopped it in half with a suitable saw, theoretically nothing negative would occur to the initial performance of said rifle? Is that what you're saying?

By taper the (end) of the barrel, it was meant as "crown". So I understand you there. I just worded it differently. As far as gas pressure goes, I know it's a factor which is why I mentioned it. It most likely is the other way around now that I think of it.

Cool your jets bro.... No need to go all high speed on me. Lol

The Army just paid me to shoot what they handed me and I was trying to help the guy out best I could. Far as I can tell the only thing I was wrong about was the gas issue. I went up instead of down. My bad.

[Sensai]

So, if you took a 12:1 twist barrel and chopped it in half with a suitable saw, theoretically nothing negative would occur to the initial performance of said rifle? Is that what you're saying?

That's not at all what I'm saying. I'm sorry that I wasn't clearer. What I'm saying is, as in your example, you took a 12:1 twist barrel and chopped it in half the bullet will still spin at the rate of one turn for every twelve inches traveled. The RPM of the bullet will be lower because the velocity will be lower. The rate of spin in reference to distance traveled will be the same, but the distance traveled per unit of time will be lower. Hope that makes sense.

As to the original question about shortening the barrel in a gas operated system, the time that the pressure is applied to the piston or impinges on the bolt carrier or action bar is lessened by shortening the length of barrel past the gas port. Once the bullet leaves the barrel the pressure drops to atmospheric. The pressure aplied is the same, but the duration is too short to give the mechanism time to overcome inertia. It's like the difference of hitting something with a two pound steel hammer and a two pound dead blow hammer. The only practical way of compensating for the lost time is to increase the volume of gas by opening the gas port(s). That is a very touchy calculation because the operating mechanism parts are not built to take sustained full barrel pressure. The short and sweet of it is, if you want a shorter barrel in a gas operated system it's best to buy one that was designed that way.

I hope that I didn't come across as too terse. Even an old numbskull like me learns a few thing if he does it long enough. I know this stuff like I know my kid's names. I just get a little carried away sometimes.

[gunderwood]

That's not at all what I'm saying. I'm sorry that I wasn't clearer. What I'm saying is, as in your example, you took a 12:1 twist barrel and chopped it in half the bullet will still spin at the rate of one turn for every twelve inches traveled. The RPM of the bullet will be lower because the velocity will be lower. The rate of spin in reference to distance traveled will be the same, but the distance traveled per unit of time will be lower. Hope that makes sense.

Exactly...except for small arms it should be 1:12 (one turn in twelve inches vice the 12 turns in one inch you speced).

As to the original question about shortening the barrel in a gas operated system, the time that the pressure is applied to the piston or impinges on the bolt carrier or action bar is lessened by shortening the length of barrel past the gas port. Once the bullet leaves the barrel the pressure drops to atmospheric. The pressure aplied is the same, but the duration is too short to give the mechanism time to overcome inertia. It's like the difference of hitting something with a two pound steel hammer and a two pound dead blow hammer. The only practical way of compensating for the lost time is to increase the volume of gas by opening the gas port(s). That is a very touchy calculation because the operating mechanism parts are not built to take sustained full barrel pressure. The short and sweet of it is, if you want a shorter barrel in a gas operated system it's best to buy one that was designed that way.

Yes, if you want your semi-auto to function properly get a barrel/gas system that is designed to work together. Even if it somewhat works (e.g. AR15 carbine gas system on a 16" barrel to meet NFA rules) there are side affects, such as the propensity to shear locking lugs on bolts at a statistically significant rate over a real 14.5" barrel/carbine gas system or the properly designed "mid-length" gas system on a 16" barrel. Also, if you don't it will drive you mad trying to find the perfect buffer/spring/whatever to make the thing work just right...until you try different ammo and it stops working again.


To the OP
What exactly is your setup and what do you want to do to it?

[kzchopper]

Just my .02 here but considering the ar platform is MODULAR by design you could just get another upper and then have a choice for which one you're in the mood for. The neat thing about a modular system is you can re-configure till you're happy with it and still change your mind later without permanantley altering anything. I would suggest having a bolt carrier for each upper as these two parts "ware" together and control headspace.

[Sensai]

Thanks, gunderwood, for catching that 12:1 vs 1:12 transposition. Something didn't feel quite right, but I didn't take time to run it down and just used the questions term. Mia culpa.

My point in all of this is that, for the vast majority of firearms users, it's much better to let the manufacturers and designers work out the major firearms modifications. Some of them are much more complex than they first seem, and can have dangerous results if not properly conducted.

[JLSCustoms]

Probably cheapest to rebarrel your AR rather than modify the existing one. Just make sure you have the right tools, i.e. armorer's wrench and torque wrench.