Hi everybody.  I just installed an Adams Arms 300 blk piston kit into my PSA AR pistol.  The instructions say to adjust the location of the gas block so that the distance between the piston cup and the gas block is between .015 and .025 inch.  I have the gas block all the way back against the barrel shoulder and I still have a gap of .055 to .060 inch.  Is this going to be a problem?  If so, does anyone know what I need to do to fix it?

David

Yes, that will be a problem. I would make sure the piston slides freely and is not binding and giving you a false reading. Then, I would call Adams Arms. They have always been helpful for me.

The piston slides as it should.   If Adams Arms doesn't respond on here, I'll give them a call when I get some time.

There is no mil Spec for Pistol Length systems. That said, we have found some pistol length, 300 BO barrels to have variations in shoulder specs. You may need to remove some of the material from the end of the drive rod in order to allow the necessary gap between the drive rod and carrier.

You may shorten the drive rod to achieve the correct clearance where it contacts the Bolt carrier. Sand and re-polish the end on a belt sander for the distance you need. There should be spring pre tension as this holds the drive rod against the gas block. The .020 gap is measured by holding the carrier into battery and moving the drive rod to contact the bolt carrier. The gap should be at the front of the drive rod after you compress the spring and make the drive rod touch the carrier while the carrier is in battery.

The distance variances happens because there is no mil spec for the gas shoulder on pistol length barrels so they are made to different lengths, which is usually taken up by the gas tube insertion into the carrier for the few hundred thousandths difference. On the piston system this needs to be addressed if the barrels shoulder is made too far back towards the receiver.

If you have any other questions, feel free to shoot an email to [email protected], or directly to me at [email protected]

-Phil

You just described the exact opposite problem of what I have.  My drive rod needs MORE length in order for the gap to be the proper .015 to .025 inch.  My gap is just under .060 inch.  The gas block is already as far back as it can get.  It is against the shoulder of the barrel.

we have tested up to .060" gap without any issues. Just to make sure, this is with the bolt carrier group installed and pushed as far forward as possible?

Yes.  What are possible problems if the gap is too large?

there should not be any problems, you have a higher likelihood of bending a drive rod, if there was no gap. but with the higher gaps that we have tested with, we have not seen any issues.

That's good to hear.  I'll give it a shot when I get my Samson handguard installed.   Thanks Phil.

sorry misread the measurements.