Just finished a 10.5" barrel build with a Ultimate Arms Gear USA Made Model 4/15 Aluminum Kit Fits SIG SAUER TAC SB15 Brace. What weight of buffer will give me best performance with this setup?

No way to know till you shoot it.

- OS

So if I use a stock weight one and have problems with ejection then step up to the next heavier one?

Maybe. Too many variables to say right off. See what happens. If 100% functional with all loads you want to shoot, and good 3:30-5:00 ejection pattern, you're probably done.

- OS

Too much information missing ...

5.56? 300 AAC?
AR15 or M16 BCG?
Gas or Piston?
Gas port size in the barrel?
etc

If you understand how it all works, you can probably do a "best guess" and it will work.

Or more importantly; does it show any signs of being over or under gassed?

If felt recoil and possibly unrelated malfunctions aren't an easy indicator (which I think they aren't easy to use), then I'd look at the ejection pattern. Two to three o'clock and I'd leave it alone.

Ar15 .223 Wylde, gas .Ejects 75% of the brass so far after 200 rnds through it but have to look at the brass yet to determine what it is doing wrong.

Is it short stroking?  What is the gas port size?  Sounds like it might need to be drilled larger.

Port is 3/32" and yes it is short stroking. What size should the port be?

3/32 is ~.094. I had to open my 10.5" build up to .101 before it cycled 100%. YMMV, depending on ammo, buffer weight, action spring strength, etc.

If you drill out the gas port, do it a few thousandths at a time, and use a drill press. Don't push the drill too hard; removing a broken drill from the gas port is a PITA, and the harder you push, the bugger the burrs you'll get in the bore.

That will be my next move I guess.Thanks.

Drilling a larger gas port hole is the last thing you should consider before checking several things first.

Drilling Port Not Recommended! Last resort modification.

Clean and lube generously the pistol, using a chamber brush and CLP.

So does it lock back on the last round fired with mag inserted and do it consistently?  What kind of "brass" ammo and who made it?  Below are some things to check before drilling the gas port.

Gas block alignment with the port and check for leaks.

Pressure test the BCG key, hold the bolt in and spray a good amount of CLP on the key and then hopefully you have an air compressor, pressurize it to about 100 pounds and look for bubbles under the key base.  If it is leaking you will need to buy new proper bolts and remount it.  Or send it back to manufacture for replacement.

Check for the BCG binding as it cycles.  You can usually feel it when you manual pull back on the handle and in the last inch or so it may start to feel harder to pull.  If it does that will need to be addressed.

Check when you manually pull all the way back on the handle where the bolt face stops in relation to the rear edge of the ejection port.  It should stop short by a 1/8" to 1/4" in front of the back edge of the port.  If it doesn't the easy fix is to remove the buffer and recoil spring and drop 2 or 3 quarters in the bottom of the receiver extension then reinsert spring and buffer.  If the face is even or disappears into the receiver, this can cause all sorts of jams.

eta: My two 10.5's gas ports are .076 and .078 and several others I've checked are similar, just saying.

And they are probably pistol-length gas, not carbine.

My 10.5" build has carbine gas, and the port was .076 originally. The bolt would only go about halfway back with .223 ammo and a standard carbine buffer. The fired case would stay in the chamber. I opened it up to about .090 and it would fail to eject about 1 round in 3. I then opened it up to .101 and it runs 100% now.

Carbine 10.5" guns need a larger gas port because there isn't much dwell time before the bullet is out of the barrel.

Sounds like good advice since I can't un-drill the port. I will try what LastRites listed and see if that cures the problem first.From what I have read elsewhere the port may already be over-sized. I was shooting American Tactical 5.56x45mm 63gr. ammo with pmags. It was worse with aluminum mag. From what I recall it did lock back on the last round.
I also had a couple of light strikes, but when examining the spent rounds they were indented correctly where the ftf ones had much less of an indent. Should I just change ammo or install a heaver spring? This all on the 1st trip to the range.

Listen to LastRites please.

Oh hell no they are not pistol length gas systems and I can only remember once where a retailer advertised a 10.5" as pistol gas and I recall the advertisement was wrong.  Since the odds are that there maybe a barrel manufacture that makes a pistol length 10.5, please show me where they can be bought.  I'd venture to bet that if you are lucky enough to find one, then try finding two and I doubt that you can as next to no one makes that configuration.

At .090+ the barrel would be on the larger size for a 5.56 barrel and would very likely be over gassing.  Your cure for your build of one, is likely masking the real reason why you were having issues.  Your port size is something that you may find on a 300 Blk Out not optimal for a 5.56 and what your giving the OP is really bad advise.

Well...being a 10.5, I'm guessing your running a carbine length gas system.

My 16" carbine likes an H1 buffer. My 18" mid does not. My 7" pistol will short stroke with anything outside of a standard buffer...so I guess it is up to the individual rifle.

With that said, if your short stroking...try a lighter buffer. If that doesn't work...I guess try an adjustable gas block.

Yes mine has a carbine length gas tube. The bolt was even with the ejection port so I will try the $0.50 cure 1st.

You never did reply to the question of whether or not it was locking back on the last round shot while a mag was inserted?  And if so it does it consistently.

Nope.
http://www.andersonrifles.com/product/10-5-m4-barrel/
Carbine gas, .086 port

https://www.classicfirearms.com/arbarrel10-5stainlesshbar1in8twist
Carbine gas, .093 port

http://www.practicalmachinist.com/vb/gunsmithing/ar15-gas-port-specs-179364/
Colt 10.5" carbine barrels have .093 gas ports also

I agree that drilling out the gas port should be done after eliminating other issues--make sure the gun is properly lubed, nothing is sticking/binding, check gas block alignment, check gas key, use standard buffer instead of H-series, etc. But if none of that solves the short-stroking problem, then drilling out the gas port is the only remaining option. And .093 is not an unreasonable size for a 10.5" carbine port.

He likes big ports and he cannot lie...

I was recently told the numbers on practical machinist for the Colt 10.5 is incorrect.  I believe they are in the .073 range for the 10.5.  



I believe the DD MK18's for .mil are in the .07 range and the civilian versions are closer to .09.  I have a 10.5" barrel coming from GM pretty soon, I'll try and measure it before I put it together.



I would not enlarge a .093 port without trying to track down why it's not working first.  You need to try it with one round and see if it locks back after the round is fired.  If it's locking back every time then you should have plenty of gas.

As for locking back when last round is fired, if I remember correctly it did several times but I was having so much problem with ejecting most rounds and spent most of my time pulling the handle I don't remember for sure. Had one round get the brass lodged above the bolt.

Sounds like an extractor issue. What brand is your bolt/extractor? Which springs did they use and which inserts?



It sounds like a Viton O-ring might help.



When you say it was short stroking, was it ever loading a second round on top of the fired case?

Besides the O-ring above.

Did you give it a through cleaning and lubing prior to firing it?  If not that would have been the first thing you should have done.

Did you check the location of the bolt face and the back edge of the ejection port when you pull back on the charging handle, all the way?