AR-15 Barrel shot group issues

[ThePunisher'sArmory]

I bought a Bushmaster XM-15 E2S M4 style carbine with what appears to be a Bushmaster AR-AK upper with the heavy barrel and an AK-74 style compensator. The problem im having is that my shot groups open up the more rounds fired and the more the barrel heats up. Also to note my gun was resting on the bipod in the same spot the whole time no changes from shot to shot. It also does this with any ammo. shot groups open up from 1.5" @ 75yrds to about 4" @75yrds after 20 rounds. The bore and barrel rifling look fine to me but i am no gunsmith by any means. I bought it used for a hell of a deal about $820, after high illinois taxes, last year. Does anyone know what could be wrong, does it sound like my barrel is "shot out" and need a new one. Please help.

 

[praharin]

Sounds about right to me. What were you expecting?

 

[rcmodel]

A non-free floated barrel, with the rifles weight resting on a bi-pod?

Yea, that grouping not out of the question at all.

Dump the bi-pod and try bench-resting the rifle on sandbags closer to the mag well.
That will take most of the weight off the forend and stop the barrel flexing so much.

rc

 

[ThePunisher'sArmory]

Good call rcmodel ill have to try that and see if the groups still get progressively larger if so ill just have to build a new upper, darn the luck. oh and paraharin i just bought it for a good cqb gun to shoot combat with, and that it still is.

 

[rcmodel]

No reason you can't just put a free-float handguard on the upper you have.

It may or may not cure the heating problem if your barrel has internal stresses in it from manufacturing.

I'd try the sandbags first though, and see what happens.

rc

 

[Nhsport]

Free floats will help to get that extra bit of accuracy out of a match gun when one is really winding ones self up in a competition sling but for a general purpose rifle not so much.

As some others have said I will generally shoot much better groups off of a rest than a bipod. Even useing a pack or a rolled towel I get better groups than a bipod,I just never did well with one.

Just to throw out another variable do a good cleaning job with a heavy duty copper solvent, then do some more testing.

To tell the truth most bushmaster barrels I have shot have had very little shift
as they warm up , the AR design seems to be less at risk for these problems than you average sporter weight rifle

Is there a bunch of crud built up on/in the compensator ?
If so take a soda can , chop the top ,fill part way with solvent and stand the compensator in it in a corner for a couple of hours alomg with a good brushing

 

[ThePunisher'sArmory]

Nhsport any recomendations on a copper solvent i usually use clp on everything and have found these bore snakes to be a good barrel cleaner, and used standard gi chamber brushes and patches.

 

[Runningman]

M4 style carbine with what appears to be a Bushmaster AR-AK upper with the heavy barrel and an AK-74 style compensator.

It will do a little better if you dump the bi pod and shoot from a proper rest. But I would not expect it to turn into a tack driver. Perhaps give it a good cleaning and maybe try some different ammo may also help you.

But....M4 style barrels are actually much closer in accuracy (due to the barrel contour) to a AR-15A1/M16A1 pencil barrels than a true heavy barrel AR. Heat tends to amplify accuracy problems on pencil barrels.

Compare a Colt 6920 M4 style barrel to a Colt 6721 heavy barrel under the hand guards and you will see what I'm talking about. Not only does the M4 style barrel have the grenade launcher cutout on the barrel. The M4 barrel profile is also much thinner under the hand guards just like the old M16 A1s. The barrel on a 16" M4 style is 1 1/2 pounds lighter than on a 16" Colt 6721 heavy barrel. End result is a M4 style barrel is more sensitive to barrel heating and barrel harmonics.

 

[Al Thompson]

TPA, I like the foaming bore cleaners. Stick an old ear plug in the muzzle and fill'er up!

 

[Double Naught Spy]

Free floats will help to get that extra bit of accuracy out of a match gun when one is really winding ones self up in a competition sling but for a general purpose rifle not so much.

Ah, but a free float will take all that pressure off the barrel added to it by the bipod and that appears to be the problem at hand.

 

[ThePunisher'sArmory]

This is a pic of my ar and the barrel

 

[joeg26er]

that looks like a free float rail - is it?
or is it a KAC RIS/RAS (or similar) system?

 

[JDGray]

The sight you have on the rail, looks like it contacts the carry handle. That, and all the other attachments, gotta go, for a sandbag test Let us know if it improves

 

[rcmodel]

+1

Looks like a mobile CQB accessory catalog!

As JDGray said, if the sight is contacting the carry handle, it's going to have to be moved so it doesn't.

Heat expansion of the upper will be pushing on the rear of the sight, and no telling where the groups will go then.

rc

 

[mec]

Mine may do that too but I never fire that many rounds without cooling the barrel down. as it is, I have a predator upper and a "Match" heavy chrome lined upper on my bushmaster ar. each will turn in groups ranging from 1.1 down to the .7 inch category wiht regularity and a variety of ammunition. Although the predator is supposed to be more accurate and wears a good hunting scope, I believe the chromed lined a2 upper with a triangel reticled acog might be just as accurate. This would negate much of the accepted wisdom about ar accuracy but much of it probably deserves to be negated.

bottom line is that both are very accurate and useful as a general utility rifle

 

[ThePunisher'sArmory]

The forearm is not free floated the unit slides into the delta ring and clamps to the front forarm holder. As for the red dot it does not touch the carry handle at all the switch is just to the left of it and the whole thing has about a 1/16" gap of space. I think it looks like it touches cause of the camera angle. Ive been doing some thinking and i think im just going to invest in a match grade rifle either a stag or another bushy with a bull barrel and free float tube. By the way thanks for all the input.