AR15 Gas Port new vs 15 rounds w/pics

[Subwrx300]

Interesting photos; I think it reinforces why there can be a need to "break in" a precision AR barrel, and possibly, why Tubbs Final Finish has helped a lot of gas rifles I've tested it on.

Here is a new match grade barrel unfired (zero rounds ever) and just cleaned. Notice there are zero burrs and port has been chamfered to remove any burrs.

Here is after 15 rounds. Cleaned twice (first shot and then after 3 shots, then shot two five shot groups, which were a bit erratic grouping and I suspected a burr/deformation had formed.

I cleaned at home and took these photos:

Bottom picture is after scrubbing port with copper remover and still has copper. It has a distinct ridge that's formed on the back edge of the port that is rasied relative to the bore, grabbing and scoring the jackets of subsequent rounds (potentially causing them the fail/separate).

This is where I think Tubbs Final Finish has a place in AR barrel break-in. I can either shoot 100-200 more rounds normally to see if erosion removes the metal OR I can use Tubbs and remove within 10-15 rounds.

Either way, I will keep an eye on this with bore cam to see how many rounds it takes to "remove" the burr if I can.

Thought some might find it interesting.

 

[Hellbender]

VERY interesting! Questions....

Was this a hand lapped barrel?
What brand?
Stainless Steel (I'm sure, but) ?
Caliber?
I assume bullet travel direction was right to left on the bottom two pics?
I've heard of the "angle drilled ports" to combat this....first I've ever seen pics of the actual problem, though!

 

[Coffee_and_Pipes]

Thanks for posting this. I’ve wondered about this issue, but don’t have the equipment to examine it. Whether you go regular ammo or Tubbs Final Finish, please keep us posted.

 

[Subwrx300]

VERY interesting! Questions....

Was this a hand lapped barrel?
What brand?
Stainless Steel (I'm sure, but) ?
Caliber?
I assume bullet travel direction was right to left on the bottom two pics?

Rainier 224 Valkyrie +3 gas system. Stainless steel and I believe they lap barrels at factory (not sure though) but finish on barrel was PERFECT from factory; looked like my Kriegers and Criterion bolt barrels.

Process:
First shot was Fed Fusion, cleaned thoroughly, 2 shots TMS rounds, cleaned thoroughly, 3 shots 90 Fusion then then 10 handloads (two five shot groups).

Zero copper in bore even before last cleaning except the gas port and one tiny spot at throat/lead. Otherwise pristine barrel and machining.

 

[AMGtuned]

Good catch! I had a gas port burr in an AR15 barrel, and could not figure out why I had so many flyers. 1300 rounds and an abundance of frustration later, I had it scoped, and found the burr. Barrel went in the trash, and I ended up with 1300 rds of brass.

 

[MSTN]

Good pix! What did you use to take them?

This phenomenon occurs in every gas port that I've ever examined with a borescope. It's most obvious - and most erratic - with new SS barrels.

Some SS barrel steel formulas exhibit extreme erosion more than others.

Sometimes, if a larger "chunk" breaks out on the forward edge, rather than a slower, progressive erosion, accuracy will immediately fall off - badly. But, that accuracy can slowly return with more rounds fired as the jagged edge is eroded smooth.

Gas port erosion continues for the entire life of a barrel. One Mid-South Shooting instructor with some crazy amount of rounds though his 5.56 Mk 18 - several tens of thousands - had what resembled a worm hole in a piece of wood tracking an inch forward of the gas port. There was no rifling for the first couple of inches, either. It was amazing the gun still ran and provided acceptable accuracy.

You don't have to have a borescope to observe extreme gas port erosion. A telltale sign of this problem can typically be seen at the crown. Clean the carbon out of the barrel and examine the crown closely, with good light. If you see one distinct, narrow streak of copper at the crown, gas port erosion is the cause.

I don't think that Tubb's Final Finish will hurt. It would be interesting to see, if you decide to try it.

I use KG Industries products on my barrels, and I believe you can accomplish much the same thing as the Final Finish (but with less wear and tear on the barrel) using KG-2 polish on two tandem VFG felt pellets. KG-2 will blend in the rough edge to a degree but not completely remove it.

 

[Subwrx300]

Good pix! What did you use to take them?

This phenomenon occurs in every gas port that I've ever examined with a borescope. It's most obvious - and most erratic - with new SS barrels.

Some SS barrel steel formulas exhibit extreme erosion more than others.

Sometimes, if a larger "chunk" breaks out on the forward edge, rather than a slower, progressive erosion, accuracy will immediately fall off - badly. But, that accuracy can slowly return with more rounds fired as the jagged edge is eroded smooth.

Gas port erosion continues for the entire life of a barrel. One Mid-South Shooting instructor with some crazy amount of rounds though his 5.56 Mk 18 - several tens of thousands - had what resembled a worm hole in a piece of wood tracking an inch forward of the gas port. There was no rifling for the first couple of inches, either. It was amazing the gun still ran and provided acceptable accuracy.

You don't have to have a borescope to observe extreme gas port erosion. A telltale sign of this problem can typically be seen at the crown. Clean the carbon out of the barrel and examine the crown closely, with good light. If you see one distinct, narrow streak of copper at the crown, gas port erosion is the cause.

I don't think that Tubb's Final Finish will hurt. It would be interesting to see, if you decide to try it.

I use KG Industries products on my barrels, and I believe you can accomplish much the same thing as the Final Finish (but with less wear and tear on the barrel) using KG-2 polish on two tandem VFG felt pellets. KG-2 will blend in the rough edge to a degree but not completely remove it.

I used a Lyman Borescope to take photos. Pretty good for the cost and my needs.

I have JB Paste and have looked at others but realistically, the best fix would be a lead lap. The VFG system works well too (I use it for both regular cleaning and JB Paste when necessary).

I just wish there was a sure fire way to remove with minimal wear and ensure even results. Anything done by stroking barrel while removing metal makes me nervous, as it quickly becomes a skill based fix versus a simple process.

Will definitely keep you all posted but I agree that this is normal in SS barrels and simply part of the process to break in barrel. My Nitrided barrels seem to have less of this , which may be a good reason to build hand lapped barrels and then have Nitrided/QPQ'd by a good company.

 

[ShooterwithNoName]

I used Tubb FF kit on a cheap Stoner SDM-R barrel. I know these barrels are hit or miss, but at the price it was worth the experiment. After running the Tubb FF kit through it, I can clean it with about 2 runs of a patch. Super easy to clean, I also can only barely detect the gas port with a patch now. Where before it would really snag the patch as it passed the gas port.

I'm very interested to see how it affects that gas port once the FF kit is ran through it. Great thread and pics, tagged.