--------------------

USMC Recon

The HK roller locking design is a bit different than other rifle designs. The chambers on most of the ones I've seen have been short by about .008" or so, which is I think normal for these due to the way they lock up. When measured on an empty chamber, the bolt gap on these rifles should be between .004" and .018". This gap is the result of the bolt head locking against the breach end of the barrel, which causes the rollers on either side of the bolt head to lock against the trunion. The rollers engage a tapered locking piece which is attached to the bolt carrier. Depending on how far the barrel protrudes into the trunion, the rollers will act on the tapered locking piece to keep the bolt gap within the desired range. So bolt gap and headspace are really two entirely different things. The two are related though, on a loaded chamber, the bolt gap will increase because the short chamber design cases the bolt head to lock up against the base of the cartridge rather than the breach end of the barrel. The taper on the locking piece is a 4 to 1, meaning that every movement inward of the rollers will result in 4 times that amount in rearward movement of the bolt carrier. So a chamber that is short by .008" will result in an increase in bolt gap by .032". Heres a pic that shows a cutaway of the HK action:

Unfortunately the little bits and pieces aren't labled, but you can see all of the important ones. Surrounding the cartridge is of course the barrel, surrounding that is the trunion. At the base of the cartridge is the bolt head, one of the rollers can be seen locked into the recessed area in the trunion. On the opposite side of the rooler is the locking piece, barely visible. Just behind that, about a third of the length back from the front edge of the mag well is where bolt gap is measured, between the bolt head and bolt carrier.

So to make a long story short, don't worry about headspace. As long as your bolt gap is between .004" and .018" on an empty chamber you should be good to go. Just to be safe, I'd measure it every hundred rounds or so for the first 1000 rounds. If you don't see more than a few thousandths loss of gap during that time you should be OK. Some of the PTR's seem to have come with faulty trunions that distort over time and allow the bolt gap to diminish very rapidly. There is a thread on this in the general topics forum as well: PTR91 Bolt Gap?

Hope this helps,

Mike

--------------------

"Helping others to find the products they need, from sources other than Joeken since 2005"

quote:

---

Originally posted by Gasman:
great info and pic, can you make that picture bigger ? thanks

---

I found that picture on google, unfortunately thats as big as it gets. Found these on HKPRO.com though, they should help explain better than words will how the HK 91 action works:

HKPRO.com

--------------------

"Helping others to find the products they need, from sources other than Joeken since 2005"

Very good diagrams of the roller operation on this page. Hope this helps,

Mike

--------------------

"Helping others to find the products they need, from sources other than Joeken since 2005"