FSB to barrel, loose fit. Is this normal?

johnson · July 22, 2011, 2:03am

In my extremely limited experience, others have had some friction when sliding on/off the FSB or block.

http://www.youtube.com/watch?v=UUY_qvSVt3w

Eric_D · July 22, 2011, 2:10am

K.L_Davis:

The gas block – The front sight, gas block assembly should be looked at to make sure that it has not moved and is not leaking. If you have a cross pin mounted setup, most likely it has not moved, but the set screw type blocks can move (usually forward) or twist and block off the gas port. Carefully check to see if it looks like the gas block has moved, you can usually see marks that indicate it has moved… it only takes an eighth inch to cause problems.

Also check for leaking, the most common sign is a brownish, rust looking stain around the barrel where it passes through the gas block. Some people will tell you that this is normal and will “seal” with time – it may be normal, but it is not right. The TDP specs for the barrel to gas block fit are one of the most exact measurement on the gun, this is a zero tolerance fit and is not supposed to leak at all – if you are getting leaking here, I strongly suggest talking to the supplier or builder about making it right.

I too have seen a fair amount of friction between barrel and fsb. I had to use a rubber mallet to dislodge the fsb on my middy when it was factory new.

If the fsb doesn’t move when pinned, and the parts function correctly and do not leak I’d say you’re gtg.

SomeOtherGuy · July 22, 2011, 2:15am

I recently had reason to take the FSB (GI A2 type) off of two brand-new Daniel Defense uppers and in each case it removed easily after the taper pins were out. I did not find any real looseness, and definitely no slop, but I was able to remove the bases with modest hand pressure and no hammer or other force. I’ve reinstalled one of them and again, no force required. Now that its taper pins are reinstalled it is 100% tight and secure. I would judge the fit of these two blocks as just having a tiny bit of clearance making it looser than a true interference fit - but the clearance is smaller than anything I can measure. As a pure WAG I would guess maybe 0.001".

johnson · July 22, 2011, 2:45am

I can rock the sight back and forth a tiny bit without the pins in place. Takes almost no pressure (gravity will move it) to slide it in and out.

I can’t test shoot it because I had to send my DD barrel nut back to them for an issue. Probably won’t get it back till next week at the earliest.

Tweak · July 22, 2011, 3:19am

As an example the A1 barrel’s (5/8") FSB seat is 0.6238 +/- 0.0005" and the A1 FSB has a 0.6249 + 0.0005" hole. At its widest a 0.0021" gap. At its narrowest 0.0006"

One of the most important steps in removing an FSB is spinning the FSB around the barrel several times after driving out the pins. Helps to break the burrs loose. A little grease on the seat and from the seat to the muzzle aids removal and prevents scuffs in the finish.