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Posted: 9/7/2013 11:20:56 AM EDT
I recently built my first 300aac blk rifle. Having a problem trouble shooting.
built on a frontier arms lower with CMMG parts.
Surplus ammo and arms buffer tube/carbine 37 coil spring and carbine buffer
barrel is Surplus ammo 300 aac
all upper parts are dpms
furniture is all Magpul

The lower functions fine on all my Ar15 biulds
when I try the 300 upper there is a failure to feed or the bolt does not completely reset. Meaning the first round fires but the second round a hear a audible click. If I push my safety selector slightly forward the bolt will set.
Is my gas tube not proper alligned? Is the gas tube not the right size? Should I be using a certain gas tube for 300 aac? The gas block is a DPMS quad gas block. Should I be using a different size on a carbine barrel?

I used the the Competitive Ar15 Biulders Guide by Glen D. Zediker and followed directions to the letter.
Really at a loss on the next step to take to make this gun function properly.
Please any input will be appreciated
Link Posted: 9/7/2013 12:43:03 PM EDT
[Last Edit: 9/7/2013 12:46:18 PM EDT by JC10311]
Welcome to the site.

I doubt that your gas system has anything to do with the problem if moving the safety selector releases the bolt and lets it go into battery.

If you manually charge the rifle, does the bolt fully close?  Same question when you use the bolt release.

When you say that the bolt sets when you move the safety selector, is the bolt held open or partially forward?  I can't immediately think of any interaction between the bolt and lower parts that would cause this.  I've done about a dozen 300 BLK builds and haven't heard of this one.

ETA: When you look into the upper with the bolt removed, does the gas tube appear to be centered in the upper?  A pic of this might help.
Link Posted: 9/7/2013 2:19:02 PM EDT
[Last Edit: 9/7/2013 2:24:06 PM EDT by oldsarg]
is it factory ammo or hand loads?
is the upper receiver assembly factory?
if you change brands of ammo is the problem there?
thinking about it have some one check and make sure  all of the trigger springs, you may have switched the  bolt lock detent spring for the one under the trigger shear,
Link Posted: 9/7/2013 3:03:27 PM EDT
I took my firing pin out and tried running my bolt several times on a mag of dummy rounds and it seems that 3 out of 10 times the bolt does not advance forward as it should. Is this a break in issue? The upper that I purchased is a factory blemish from SAA and the BCG is DPMS. Maybe they do not mesh well, I doubt that but could be. My gas tube appears to be centered in the upper.
I have only ran 30 rounds thru the gun.
Soon as I can figure out how to load pics I will
Link Posted: 9/7/2013 3:05:47 PM EDT
All factory ammo.
The lower functions perfectly on my other 3 ar15 5.56 rifles.
The upper is a SAA barrel  and a SAA upper receiver that I assembled.
Link Posted: 9/7/2013 3:11:27 PM EDT
[Last Edit: 9/7/2013 3:11:52 PM EDT by JC10311]
Have you tried the 300 BLK upper on another lower?  To load pics, you need an account on Photobucket or another photo sharing site.  Then you can add a link to the pic here.
Link Posted: 9/7/2013 3:37:34 PM EDT
Yes I did try swapping uppers. I took my best Ar15 and swapped the lower onto the 300 upper and put the 300 upper on the ar15 lower. That is how I decided I might have a gas tube issue since the same failure happened on a known good lower.
http://s1285.photobucket.com/user/bleefedex/library/

Let me know if that works for photobucket
I am clueless with these computers and my teenage daughter is not home
Link Posted: 9/7/2013 3:38:55 PM EDT
Link Posted: 9/7/2013 4:20:04 PM EDT
It looks like you're having trouble getting pics loaded.  If you want, e-mail them to me and I'll post until your daughter gets home.

To eliminate the gas tube from the equation, I'd remove it (by loosening and/or removing the gas block) and see if the bolt behaves correctly when manually cycled.
Link Posted: 9/7/2013 6:48:53 PM EDT
[Last Edit: 9/7/2013 6:50:37 PM EDT by vellnueve]
Try using the BCG out of another rifle in it.

What ammo are you using?
Link Posted: 9/7/2013 7:13:49 PM EDT
Hornady 110grn and 208grn Also ran a couple reloads with sierra 110grn
Link Posted: 9/7/2013 9:36:08 PM EDT
That gas tube looks perfect.  I agree with the above suggestion about using a different BCG.
Link Posted: 9/8/2013 11:47:20 AM EDT
I will try a new BCG an next trip to range hopefully this week.
Link Posted: 9/8/2013 12:20:30 PM EDT
What's buffer's and springs are you running?
Link Posted: 9/8/2013 7:22:19 PM EDT
Not sure if its a brand name. I purchased the tube spring and buffer at a gun show from a local Gun dealer out of Lakewood, Wa. Surplus Arms and Ammo. I believe they have there own line of parts.
I compared the spring and buffer to my other builds and they are the same other then 1 or 2 coils longer.
Link Posted: 9/8/2013 7:28:46 PM EDT
[Last Edit: 9/8/2013 7:31:54 PM EDT by talonxracer]
Are you running the weapon wet or dry, they love to be soaking wet, I got a quart of synthetic 0w20 motor oil and liberally douche the BCG before going out back shooting. The pistol powders used in the 300blk are very dirty running powders.

If you have any other carbine buffer setups give it a try.

What does the chamber look like? Rough or smooth?
Link Posted: 9/9/2013 4:42:00 AM EDT
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By bleefedex:
Not sure if its a brand name. I purchased the tube spring and buffer at a gun show from a local Gun dealer out of Lakewood, Wa. Surplus Arms and Ammo. I believe they have there own line of parts.
I compared the spring and buffer to my other builds and they are the same other then 1 or 2 coils longer.
View Quote


You do realize that buffers vary by weight, not length?
Link Posted: 9/9/2013 7:41:31 AM EDT
Ya I was aware about the spring and buffer differences. I asked the sales person before purchase to make sure the setup would work in a carbine and that they would function with the caliber since this was a new build for me. The only difference between my other buffer/springs is the new spring appears to be not as compressed as my other biulds, but same count on coils. Like I stated earlier that the lower functions perfectly on all my other configurations.
I have to say that I have not ran it wet yet.
This week I will try soaking the BCG and running a different BCG if I get the same results.
Link Posted: 9/9/2013 5:31:06 PM EDT
The barrel ramp doesnt look like its lining up with the ramps on the reciever. You have got to get the right extensions to go with your upper reciever.
Then again it could just be the light in the picture.

Link Posted: 9/10/2013 5:04:43 AM EDT
Does the bcg lock back on an empty mag after firing?

Stick with factory super sonic ammo, sub's are known to cause issues without a can.

Also did you really mean the safety or forward assist?
Link Posted: 9/10/2013 2:04:23 PM EDT
safety. When I noticed after firing, the second round just clicked I put the safety on to figure out the problem. That's when I heard a click sound in the lower. Moving the safety selector must have engaged the bolt all the way forward.
Link Posted: 9/10/2013 2:18:08 PM EDT
Chamber a round and have an empty mag in place to see if it locks open.  You really need yo just start eliminating possible issues.

Be careful you don't want a round going off without the bolt fully closed.
Link Posted: 9/10/2013 3:09:14 PM EDT
[Last Edit: 9/10/2013 3:09:43 PM EDT by talonxracer]
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By bleefedex:
safety. When I noticed after firing, the second round just clicked I put the safety on to figure out the problem. That's when I heard a click sound in the lower. Moving the safety selector must have engaged the bolt all the way forward.
View Quote


That sounds like a disconnector timing issue
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