Muzzle expansion for 1/2x28 tpi threads?

[Aloha_Shooter]

I've just ordered parts for 3 projects but as I was selecting barrels, I came across comments about how 1/2x28 tpi threads allow muzzle expansion which creates inaccuracies and some barrel makers were crowning or using a different thread pattern for the muzzle. All three projects were to be finished with AAC Brakeout 51T flash hiders that I'd already purchased with 1/2x28 threads. One of the projects is intended to be a varmint rifle so I was going with a 20+ inch barrel (finally settled on the YHM 77TF -- 20 inches, melonited, fluted, and threaded at the muzzle with 1/2x28) but I'm wondering if I set myself up for failure with this and should have gone with a crowned or alternate thread pattern on the muzzle.

I'm curious about the claims WRT muzzle expansion. Wouldn't the muzzle device prevent this?

FWIW, the other two projects are a 16-inch carbine and an 18-inch SPR. It probably would have been cheaper to just order ready-made uppers but this will (hopefully) force me to learn more about the details of each rifle.

[MarkCO]

Are you going to be shooting for sub 1/4MOA groups? If not, don't worry about it. My 18" match rifles shoot down to about 1/2" MOA and are all 1/2x28 threaded.

The fluting will have a greater detrimental effect than the 1/2x28 threads.

[fullmann]

I know I read about muzzle expansion on the 6.8's that were threaded 1/2x28.

first I've heard of it on 556/223 though.

[davsel]

I believe the "muzzle expansion" happens when the threads are cut into the new barrel - stress relief.

[BPTactical]

Many factors at play here.
On a 6.8 I could possibly see it as you start getting a pretty thin wall.
On a button rifled barrel in 5.56 maybe, when the rifling is done it is by displacement of material from the "button" being pressed through the barrel. This creates substantial internal stress factors.
Cut rifling? Doubtful. The grooves are physically cut into the barrel and while you have some stress from the process I would doubt it would manifest itself by the muzzle "opening up".
Forged barrel? I would not think so. The grain structure is so compacted in a forged barrel.
What I see having a greater effect is the tension placed on the turned shank by tightening the muzzle device excessively.
The force generated by a threaded fastener (essentially what the muzzle device is achieving) would be a far greater factor than whatever stresses are present.

[Aloha_Shooter]

Thanks, I was curious as I saw the following statement at ar15performance.com (among other places):

[QUOTE]NOTICE*** THIS BARREL HAS 5/8-24 threads to keep the bore at the muzzle from swelling like most barrels threaded 1/2-28. The tighter bore at the crown gives better accuracy.[/QUOTE]

Things are what they are as I ordered parts to take advantage of the July 4th sales. In fact, the 16-inch barrel just arrived ... too bad the upper for it is on backorder ...

[BPTactical]

[QUOTE]NOTICE*** THIS BARREL HAS 5/8-24 threads to keep the bore at the muzzle from swelling like most barrels threaded 1/2-28. The tighter bore at the crown gives better accuracy.

Snake oil.
The design has been in service for 50+ years.
If it was such a problem we would have heard about it long ago.
The serious precision rifle guys don't stress the diameters that I am aware of.
They worry more about the torque of the muzzle device for the reason I mentioned.

More than likely a manufacturer screwed the poochy and ran an ass load of barrels with the wrong thread profile.

[Great-Kazoo]

Snake oil.



More than likely a manufacturer screwed the poochy and ran an ass load of barrels with the wrong thread profile.

Probably has a boat load of flash hiders and suppressor mounts he needs to move also. Almost Free Amigo

[Aloha_Shooter]

Yeah, that's what I figured so I went ahead and ordered what I did. I was just wondering in hindsight if anyone had any info on the allegation.

By the way, Bert, how's your workload looking?

[Skullworks]

I think the key thing here is to not fire the weapon with the threads exposed - if a flashhider/break is installed and torqued to spec those threads will be fully supported and I would have no worries.