First AR build - rough on cases

[BumbleBug]

I completed my first AR build using all PSA parts including a complete upper they sell. The upper is a PTAC 16" 1/7. I temporarily mounted some high-powered Leupold glass for initial testing. I'm a dyed-in-the-wool handloader but since this was the AR's first time out, I bought a box of Win 55gr FMJ to assure proper function. Note the pictures below. The ejected cases were greatly scratched (circularly) & some frosty looking spots. The rifle shot fine & never missed a beat to function/fire/eject. I also made up some handloads that functioned perfectly & they showed the same case scratching. BTW: the accuracy off the bench was between 3" & 4" at 100yds.

• Am I right assuming that I need to polish the chamber?
• Do I have to disassemble the upper to do the polishing?
• If I have to disassemble & polish am I wasting my time - just get another barrel?
• Or put some open sights on it & call it a day as an AR plinker?

TIA...

..bug

 

[Theohazard]

Do you have a carbine-length gas system or a mid-length? What buffer weight are you using? It sounds like your rifle is over-gassed and is extracting the case early while it's still expanded against the chamber wall. An easy fix to this is a heavier buffer, which slows down the bolt's unlocking until pressure has dropped more.

 

[BumbleBug]

Thanks for the response Theohazard.

I'm not sure how to answer your questions. I went back to the PSA website to look up exactly what I bought, but they lost their records before April of 2014??? But here is what I know.

The barrel is indeed 16" from firing-pin end of chamber to to crown (not the flash hider). The center of the gas block is located 8-3/8" back from the crown. Mid length? The buffer weight weighs exactly 3oz & has a back-n-forth rattle.

TIA for the help!

 

[Theohazard]

No problem! OK, you have a carbine-length gas system. With a 16" barrel, that gives you a little more gas flow than you need (the M4 also has a carbine-length gas system, but with its 14.5" barrel it has less dwell time and therefore less gas coming through the gas tube).

A mid-length gas system gives the ideal gas flow with a 16" barrel. But instead of buying a new barrel and gas tube, an easy way to counter your extra gas flow is with a heavier buffer, but it looks like you have a standard carbine-weight buffer. An "H" buffer is the next step heavier; that or an H2 buffer (one step heavier than that) will probably fix your problem. You'll know your buffer is too heavy when your rifle won't reliably lock back on the last round, or it stops locking back when it's dirty.

It's still possible you have other issues going on, but first try an H or H2 buffer and see if that fixes things before you do anything else.

 

[marine6680]

Get an H3 buffer.

If its too heavy, you can combine the H3 and standard buffer into any other buffer weight you want. Though an H3 tends to work well in a carbine gas... if using 5.56 spec ammo it should work really well. Some weak ammo may have issues though. But my mid gas runs an H2 and weak ammo with no issues, so an H3 should be fine. If it gets really dirty, you may have issues though.


The buffer has three cylindrical weights inside, that is the rattle you hear.

A standard has 3 steel weights, the H3 has 3 tungsten weights which gives it a higher total weight. You can open them up and change the combination to make other total weights.


If you still have issues, then contact PSA and they can fix you up.

 

[Theohazard]

Get an H3 buffer.

If its too heavy, you can combine the H3 and standard buffer into any other buffer weight you want. Though an H3 tends to work well in a carbine gas... if using 5.56 spec ammo it should work really well. Some weak ammo may have issues though. But my mid gas runs an H2 and weak ammo with no issues, so an H3 should be fine. If it gets really dirty, you may have issues though.

The H3 will probably be too heavy (unless he's shooting suppressed). Even if it isn't, it probably will be too heavy once it gets really dirty, like you pointed out.

I like your thinking on taking it apart and moving weights to make an H2-weight buffer, but if he doesn't want to deal with that it would be easier for him to just buy an H2 buffer and be done with it.

 

[BumbleBug]

Can I just get the tungsten plugs?

Thanks for the help & good advice. I've definitely learned something.

Searching the net I found out that the H1 weighs 3.7oz (1 tungsten & 2 steel) & the H2 weighs 4.6oz (2 tungsten & 1 steel). The H3 has 3 tungsten, but I can't find the weight.

Not sure about the internals & how the steel plugs & spacers work & how critical, but couldn't I just add a tungsten weight to my existing buffer to make it a H1? The H2 buffers I've found are around $30. I haven't found a H3 yet. I guess just the plugs are not for sale?

...bug

 

[HKFan9]

Theo and Marine are spot on, to make life easier I side with Theo.. an H2 buffer with either 556 spec or 223 spec ammo should work fine.

 

[marine6680]

I have not seen the tungsten slugs for sale individually. The H3 buffers usually run for $35 or so.


The internals of the buffer are simple.

Looking with the rubber/plastic rear plug pointing up. At the bottom of the stack is a rubber spacer, then a weight, then another spacer, weight, spacer, weight, then the rear plug.


My mid gas rifles run fine with an H2, so the carbine gas with its higher pressures should handle an H3 buffer. But I do agree that if you don't want to mess with taking one apart, an H2 is a safer bet. It will run in more varied circumstances, but may be a little lighter than ideal.


I have seen it recommended by an instructor for the military... In a carbine gas rifle with proper sized gas port in the barrel (even actual m4s as well) that using an H3 buffer, extra power spring, and using the o-ring and insert for the extractor spring... That combination running 5.56 spec ammo was said to be the best combo for the carbine gas rifle. He claimed combat and extensive training sessions had shown him it worked.

Is it the best? I don't know personally... I am just passing it along. If it does work, that shows that the rifle is very heavily gassed and can handle a lot.

For the average person using a variety of ammo, I would skip the extra power spring.


The good thing about all of it, its easy testable and reversable if testing proves the combination problematic. The downside is that it requires a little investment in cash to get the parts.

 

[marine6680]

BTW... That info is just FYI for anyone reading.


As far as the OP and his issue, a simple swap to a heavier buffer is the first step, and most likely only step needed. Most don't need to get too deep into their rifle, and those new to the platform can be overwhelmed if they try to tackle everything all at once.


I meant to ask before. What does the base of the brass look like. Are there any smears on the headstamp?

 

[BumbleBug]

The bases of the brass are clean with no smears. On the case side of the rims, there is a very slight extractor burr but it does not seem out of normal.

BTW: Is there any significance to why my buffer rattles (i.e. the weights/spacers are loose inside)?

 

[marine6680]

That is by design.

The weights and spacers are allowed a little room to move, as they work to prevent bolt bounce.

Its very similar to how a dead blow hammer works. Spikes uses tungsten powder, it can damped vibrations better than standard buffers. Not really needed over a standard buffer unless you just want it.


Hopefully a buffer fixes the issue, and you do not need to warranty the upper. But PSA should help you out if you do need to do so.

 

[BumbleBug]

Fixed! Thanks....

I ordered a H2 buffer from PSA which cost about $37 w/ship. That fixed it! Fired cases do not have any frosty scratches, just a bit sooty. The last round fired leaves the bolt open.

Thanks to all for their comments & diagnosis!

...bug

 

[marine6680]

Glad you got it working.