UPDATE: Reloading and the Headspace dilemma...
So far I've been really happy with how this rifle has turned out. A couple of little "comfort mods" have made it a little nicer to shoot, specifically a Harris 9-13" bipod and a Vortex anti-cant bubble level. I've grown tired of paying for factory match ammo though, and decided that I'd rather put that money towards some reloading gear and see if I can tighten the groups up a little more than <1.5 MOA.
That's where my headspace dilemma came in. I'm usually reluctant to mess with a winning combination, but if I was going to go to the trouble of buying and sorting decent brass I wanted to be able to get a few iterations out of it. Unfortunately, with the combination of a 1.645 headspace and small-base resizing dies, I didn't feel comfortable risking more than two or three re-loadings. Either I was going to go through a lot of brass, or I needed to tighten up that headspace.
The most practical option would have been a new barrel, but the one I had was really working for me. Instead, and despite much online advice to the contrary, I ordered a stripped Fulton Gen II bolt from treelinem14.com with the intention of lapping it to fit. While waiting for it I picked up some 240, 320 and 600-grit lapping compound and built a jig out of a resized case and an old 1911 recoil spring.
Predictably, when the bolt arrived it didn't just drop right in. I had about 3 thousandths worth of barrel protrusion that had to be carefully filed down to relieve binding on the front of the left lug, but that was a fairly straightforward fix. Once the bolt was able to close freely I found that I was sitting at about 1.624, based on being a hair short of closing on a resized case mic'd at -0.005. I wanted a comfortable 1.632-1.633 so I was looking at a LOT of lapping. If Fulton's claim of meeting mil-spec are accurate, the case-hardening layer should extend between 0.012 to 0.018, and taking 0.008 off of that sounded like a pretty bad idea. Lapping wears down both the bolt and receiver lugs though, and if they both wore at the same rate I should be left with a hardened layer that was still a minimum of 0.008 thick. Maybe not so bad. Worst case scenario is I'd ruin both the bolt and the receiver so what the hell, I figured I'd give it a shot.
Keeping track of how much was wearing off the bolt's lugs was easy, I just measured them before I started and kept re-measuring every time I stopped to try the GO gauge. I couldn't figure out a way to measure the wear on the receiver lugs, so I had to wait for the GO gauge to chamber and extrapolate by subtracting the bolt lug measurements from the overall change in headspace. When the bolt finally closed on the GO gauge I'd lost about 0.0033 from both bolt lugs so that meant I'd only lost about 0.0027 from the reveiver, but that was close enough to even wear for my book. I lapped a little more until I'd removed as close to exactly 0.004 from the bolt lugs as possible and checked the headspace gauges again. The bolt closed on the GO gauge (1.630) just by tilting it shut but hung up with about 2/3 bolt engagement on the NO-GO (1.634). Hopefully it was right around 1.632. I tried some Winchester White Box 147gr .308 in the chamber and the stripped bolt closed easily on them.
I stripped my old bolt and fit the parts onto the new one with no trouble. The firing pin protrusion measured at 0.056, which was high but still within spec. Using my thumb to keep the firing pin protruded on a closed bolt I tried lifting the bolt roller and the safety bridge stopped it at about 60% lug engagement - not great, but slightly better than the old bolt when I compared them. My next upgrade will be to order a USGI firing pin and see if the source of the problem is the pin geometry or a sloppy safety bridge.
I greased her all up and put everything back together and she passed the tilt and function tests smoothly. Now the only thing left to do was light off 55,000 or so PSI and see what happened.
Yesterday morning was cold and miserably windy, so I didn't plan on being out long. Just enough time to fire off that box of WWB and see if everything was still intact afterwards. A single round to start chambered and fired fine, then two more, then five more - all cycled and ejected properly and went pretty much where I was aiming at the back of the berm. As I was also using my new AIA mag for the first time I filled it with ten shots and cycled through them without a hiccup. For the last two I turned off the gas system and cycled them by hand, they mic'd at just under 1.632 and just over 1.632 respectively. Nice! Now I just have to see if I've changed the accuracy mojo for better or worse, but that's going to have to wait for a warmer day. For now, I'm content to stay warm and read through my brand new reloading manuals for a while
A caveat for anyone thinking of doing the same thing: the experts all tend to agree that it is FAR better and safer to cut a chamber deeper than to try to lap a bolt significantly for headspace. A new barrel tends to pay much greater dividends in terms of accuracy as well. If you still decide to try what I did, then I strongly suggest that you regularly re-check your headspace to make sure that you haven't worn the case-hardening layer too thin. I'll be checking mine pretty anally for a good while until I'm certain I'm in the clear - probably for at least the next 1000-1500 rounds. If the headspace starts to open up I'm going to need to either chuck the bolt and/or receiver or send them off to be melonited or something.
Hey Pop, man after my own heart!!! Can't resist playing with those damn Norincos..... sure would like to see your range report when you get it done!!!
Rodney
