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Discussion Starter #1
Normally with other type of rifles. I would disassemble the bolt and try some of my resized brass just to make sure as I use a drop gage to test sizing while working. My PTR (I also have a 91 same phenomena) bolt gap is at a steady .457mm. I thought well I'll just drop in a piece of brand new IMI brass (it was at the min length of the drop gage) I drop the bolt pull the trigger and look at the gap. Lo and behold the gap jumped to .610mm. I then tried a piece of sized brass that was at max length in the drop gage. It came in at .639. I don't get why the gap is growing as the brass length increases. Any ideas?
 

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Because bolt gap is checked with the bolt face against the barrel. When you put a round in, the bolt face is no longer touching the barrel. As the rollers start climbing out of the groove in the trunion, they push on the locking piece which separates the bolt from the carrier.
 

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Discussion Starter #3 (Edited)
Thanks mt162. What I don't understand is should a properly set up G3, 91 or PTR close with the bolt face against the breech? This would allow the rollers to fully extend in the trunion cutouts. If the bolt rests on the back of the case, then the rollers can't fully seat, the locking piece can't go fully forward and so the apparent bolt gap increase.

If my thinking is right then checking the bolt gap on an empty weapon will tell if the timing is correct but a prudent owner would also carefully check the gap on a loaded safetied weapon to ensure it was still less than .5 mm to know his rollers were fully engaging.

My HK 91 is less than .5mm loaded. My PTR is grows to over 5mm. Tells me the PTR has insufficient headspace.

http://www.youtube.com/watch?v=MFESb8TK7ZA&feature=user
 
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