Went to the range today with a built upper that has never been shot. Equipment is a rainier upper, sig mp400 14.5" barrel,  stock mp15 BCG, and a lower profile gas block. When I was able to get a shot off the brass ejected fine (or enough to not get stuck in the chamber) but after one or two shots it would fail to chamber the next round After I chamber the round manually it will shoot that round fine but then the bolt would go half way and not chamber the next round fully. I suspected a gas issue so I took the gas tube off and checked to see if it was crimped in anyway which it was not. Then checked the alignment of the gas block via the paper clip method and it clears into the hole in the barrel. I haven't tried compressed air into the gas tube yet but it seems to clear the other side when I blow into it by mouth. I can't seem to figure this out. My BCG worked with my old upper and I haven't touched it since I shot it last. Could something have gone wrong with the BCG not cleaning it the last time I shot? I hope its not that fragile. I did the stand the bcg on the bolt thing and it stayed up.

Couple things I'd wonder if not a gas issue is .... Are you shooting reloads? And possibly buffer weight issue? Or one of the hydraulic buffers? Was the bolt/upper BONE dry(no lube)?

What kind of gas block do you have? It is an adjustable gas? Factory ammo?

What kind of gas block do you have? It is an adjustable gas? Factory ammo?

 

yankee hill low profile. its not adjustable. I was using PMC 5.56 55gr. I'm guessing that the gun is somehow having a gas issue or its cycling too slow for some other reason. I did notice some brass color markings on my brass deflector. not sure if thats normal or not.

Those marks in the deflector are normal. Triple check the alignment on the gas block. What kind of buffer and spring are you using? What's the length of your stock? Rifle or carbine? If using rifle lenght make sure you are not using a carbine spring.

Couple things I'd wonder if not a gas issue is .... Are you shooting reloads? And possibly buffer weight issue? Or one of the hydraulic buffers? Was the bolt/upper BONE dry(no lube)?

 

I have the spikes s2-t2 buffer but changed to the stock lighter buffer with same results. No reloads, I was shooting PMC 5.56 55gr and since I haven't cleaned the bolt since the last time I shot about 3 weeks ago I would say the bolt most likely wasn't properly lubed and the upper and barrel are brand new. I did take a swab soaked with lube and wet the chamber and the barrel but not a whole lot. But would a dry BCG or upper cause this? 

Perhaps you need a lighter buffer. Are you using an H2 or rifle buffer?

Maybe you need a carbine buffer ans spring.

 

Those marks in the deflector are normal. Triple check the alignment on the gas block. What kind of buffer and spring are you using? Whta's the lenght of uour stock? Rifle or carbine? If using rifle lenght make sure you are mot using a carbone spring.

 

yep I checked the alignment and the paper clip goes into the holes just fine. I'm using stock mp15 spring with spikes s2-t2 buffer. The barrel and biffer are carbine length 

Perhaps you need a lighter buffer. Are you using an H2 or rifle buffer?

Maybe you need a carbine buffer ans spring.

 

it was shooting fine when i had my stock mp15 upper on there would a new upper and barrel change the way it should cycle?

Ejection without feeding typically points to a gas issue - although I once saw it due to an put of spec Cam pin that was binding up in the BCG and slowing it down... Your BCG is not being moved far enough to the rear to strip a new round from the mag.

 

Problems like these are common with WECSOG guns. Without going hands on, it will be difficult to diagnose. My first guess is BCG. Make sure the gas key is tight. Second guess is gas block/gas port issues where they are not lining up properly.

 

Brass on your brass deflector is 100% normal.

Fixed so you can sleep tonight

it was shooting fine when i had my stock mp15 upper on there would a new upper and barrel change the way it should cycle?

Absolutely. Your new upper is 14.5, meaning it has less gas than a 16". Try a lighter buffer. Same thing happened on my spike's midlength upper with 14.5 barrel. It was undergassed.

Absolutely. Your new upper is 14.5, meaning it has less gas than a 16". Try a lighter buffer. Same thing happened on my spike's midlength upper.

 

hmmm but with the comp its 16" not sure if that matters or not. I did try my stock buffer which is lighter than the spikes and same thing happened.

How's that paper clip method that you are using to check alignment?

Ejection without feeding typically points to a gas issue - although I once saw it due to an put of spec Cam pin that was binding up in the BCG and slowing it down... Your BCG is not being moved far enough to the rear to strip a new round from the mag.

 

Problems like these are common with WECSOG guns. Without going hands on, it will be difficult to diagnose. My first guess is BCG. Make sure the gas key is tight. Second guess is gas block/gas port issues where they are not lining up properly.

 

Brass on your brass deflector is 100% normal.

 

gas block is definitely lines up with the barrel hole and same with the gas tube to the gas block. CHecked the gas key and that sucker hasn't budged one bit and its staked. not sure if a leak is possible but it just being under gassed somehow. BTW using my logic, I ruled out a barrel to upper chamber alignment issue since there are times where the next round isn't even cycled and i get a dry fire. Could this be the culprit as well?

How's that paper clip method that you are using to check alignment?

 

I bend it at a L just long enough that it would go past the block and into the barrel and insert it into the hole at the bottom of the block. It clears both holes

I bend it at a L just long enough that it would go past the block and into the barrel and insert it into the hole at the bottom of the block. It clears both holes

Can you see the end of the paper clip through the barrel?

My experience is that there does not seem to be a standard port size for midlength uppers. Also, you can't count muzzle attachments towards barrel length when it comes to gas. There is a theory that there is an optimal distance between the end of the barrel and the gas port. 16" is optimal for midlength. 14.5" for carbine. 16" barrel on carbines are overgassed. 14.5 are undergassed on midlength.

Can you see the end of the paper clip through the barrel?

 

i didn't bend the L that long but I'll try now. Should it be able to go straight through?

i didn't bend the L that long but I'll try now. Should it be able to go straight through?

Yes. You can try with copper wire and it will bend as you push.

My experience is that there does not seem to be a standard port size for midlength uppers. Also, you can't count muzzle attachments towards barrel length when it comes to gas. There is a theory that there is an optimal distance between the end of the barrel and the gas port. 16" is optimal for midlength. 14.5" for carbine. 16" barrel on carbines are overgassed. 14.5 are undergassed on midlength.

 

the gas hole is carbine length though not midlength

if there was a gas leak would i be able to see the carbon built up somewhere from just 60-70 rounds?

if there was a gas leak would i be able to see the carbon built up somewhere from just 60-70 rounds?

Do you see any around the gas block? You should not.

Also it will not lock the bolt on the last round

Do you see any around the gas block? You should not.

 

nope, the gas tube on both ends though have carbon build up. 

nope, the gas tube on both ends though have carbon build up.

I don't think you have a leak. I more inclined to think that your gas block is a bit off center making the gas port smaller and as a result less gas for cycling

I don't think you have a leak. I more inclined to think that your gas block is a bit off center making the gas port smaller and as a result less gas for cycling

 

just tried and copper wire cleared through the barrel

Running out of ideas... Is properly lubed?

Compatibility issue with the low profile gas block, is my guess

Running out of ideas... Is properly lubed?

 

me too, and in all honesty its probably not properly lubed. I used some cleaner lubricant to line the inside of the upper and the barrel but it was a light coat. Can that really cause a cycling problem though?

me too, and in all honesty its probably not properly lubed. I used some cleaner lubricant to line the inside of the upper and the barrel but it was a light coat. Can that really cause a cycling problem though?

I think it can. Specially the first shots when all is recently put together. But this is only my guess...