Gas tube position new PSA .300 Blk upper / Sig lower & BCG

[jc-nh]

I'm hoping you all can help me out with a problem I'm having with a new upper.

I have an Sig Sauer M400 SDI in 5.56 - I bought a Palmetto State Armory .300 Blackout upper that I'm trying to run on the SDI. However, the Sig BCG doesn't quite insert all the way in the PSA upper because the gas tube extends into the key slightly further (about 1/8") than the one on the stock Sig upper.


SIG left, PSA right (note the PSA is higher in the key)



Sig left with proper BCG fit, PSA right with improper BCG fit

I'd love some advice.

Is this a BCG problem?
Is this a gas block / gas tube problem?
Is the Sig BCG just built differently than most and a new BCG would likely just solve the problem?
Is this a problem with the barrel being too far back in the receiver?
None of these?
What's the right way to resolve this?

Some things I checked:
- My wheeler engineering action rod seats and locks perfectly in this upper
- With the gas tube removed, the BCG fits perfectly in the upper
- The gas tube is fully inserted into the gas block, and the gas block is properly situated over the gas port on the barrel
- The gas tube is exactly the specified length (it's a pistol length, so 6.75")
- Without the BCG, the upper mates perfectly with the lower

Thank you in advance,
-J

 

[whatluck]

I thought sig lowers were proprietary?

 

[jc-nh]

I thought sig lowers were proprietary?

Thank you. I don't believe the lower is an issue here, as the fit with the lower seems fantastic.

 

[pastera]

300blk uses the same BCG as 223/556
Swap BCG between uppers
If failure follows the BCG - that's your issue
If it's the upper - pull the gas block and tube

Or send the upper back to have it fixed

 

[jc-nh]

300blk uses the same BCG as 223/556
Swap BCG between uppers
If failure follows the BCG - that's your issue
If it's the upper - pull the gas block and tube

Or send the upper back to have it fixed

Thank you. I can't swap BCG's, as you stated the BCG between the two calibers is the same, so with only one lower I didn't buy a second BCG.

 

[Marty McFly]

I know your screen name eludes to new hampshire but if you're actually southern mass I can borrow you another bcg to try. If it were me I would have filed the tube a hair shorter.

 

[jc-nh]

I know your screen name eludes to new hampshire but if you're actually southern mass I can borrow you another bcg to try. If it were me I would have filed the tube a hair shorter.

I appreciate the offer, if I were closer (and of the laws in that state were at all sane), I might've taken you up on that!

 

[Marty McFly]

I appreciate the offer, if I were closer (and of the laws in that state were at all sane), I might've taken you up on that!

Did you try to put the bolt in without the charging handle?
I feel like it's going to be the gas tube but if the charging handle is sliding a hair sideways?

 

[jc-nh]

The mystery is solved. Figuring another lower is in my future at some point, I just bought another (non Sig Sauer) BCG. It works just fine. Culprit, a finicky Sig BCG.

Thank you all for the help.

 

[pastera]

I don't understand not dropping a few extra bucks on a lower to have a complete rifle - doesn't need to be a top tier on every gun.

 

[Marty McFly]

I don't understand not dropping a few extra bucks on a lower to have a complete rifle - doesn't need to be a top tier on every gun.

I used to do that when I was younger and money was tighter

 

[jc-nh]

I don't understand not dropping a few extra bucks on a lower to have a complete rifle - doesn't need to be a top tier on every gun.

Will definitely do that at some point. It's not about money, it's about spending enough time with the platform to understand what I like and don't before choosing. At this point, I'm only a few thousand rounds in.

 

[pastera]

Will definitely do that at some point. It's not about money, it's about spending enough time with the platform to understand what I like and don't before choosing. At this point, I'm only a few thousand rounds in.

Get a cheap lower with mil spec parts - $115
Even with a transfer fee on the lower it's cheap but still goes bang every time
No guilt in changing out parts one by one since those parts just become the N+1 that eventually gets built