When I woke up this morning, it was reasonably cool, overcast and calm ... just perfect for a day at the farm range. I grabbed a couple 10/22s, some tools, a chronograph, and a brick of ammo. The only thing I forgot was my digital camera.
I started out chronographing 10 shot strings from my Green Mountain 20" bull barrel with the bolt that has the most headspace (.012"). My average velocity was 1271 fps and my max spread was 39 fps. I then swapped out the bolt assembly with another 10/22 that had a tighter headspace of .008". The rest of the gun was the same including the barrel. This time my average velocity went up slightly to 1286 fps and the max spread went down to 29 fps. So, this was looking good. A slight increase in velocity of 15 fps and 10 fps slightly tighter max spread.
Note: Normal headspace is .010" so .012" is .002" looser than normal headspace and .008" is .002" tighter than normal headspace.
I repeated the same exact test with my 1974 vintage 10/22 with a 18.5" factory barrel with both the same bolts as above. This time I got 1255 fps average with the tight bolt and 1242 fps with the loose bolt for a difference of 13 fps. My max spreads were 55 fps with the loose bolt and 44 fps with the tight bolt.
Based on these two tests, it's safe to conclude that in my guns, tighter headspace increased velocity by a token amount and decreased max velocity spread also by a token amount. Because the GM barrel is 1.5" longer and is built to much tighter specs, the velocity was higher than the factory 18.5" barrel and max spreads were tighter.
I set up several targets at 50 yards and started by mounting and sighting in a 4-12X varmint scope on my 1974 10/22 with the AO set to 50 yards (18.5" factory barrel). After sight-in, I fired 3 - five shot groups from a good bench rest on separate targets then changed the bolt. The loose bolt groups averaged 2 1/8" for the three targets. I repeated the same exact test with the tight bolt with nearly identical results ... One group was a token larger, one was the same and one was a token smaller. In all, they both averaged 2 1/8". POI was exactly the same on all 6 targets.
With the bull barrel 10/22, I repeated the above tests with both bolts. With the loose bolt I got a .42" average group size for 5 shots in each of 3 targets. With the tight bolt, my group average shrunk to .40". That's a whopping 20 thousandths tighter!
After a very fun day of shooting the 10/22s, I learned some amazing stuff. Tighter headspace seemed to increase muzzle velocity and tightened max velocity spread by a tad. Accuracy at 50 yards with the standard factory barrel was unchanged. Accuracy with the GM bull barrel was so close I'd call it unchanged too. Based on my own tests, I'd have to say spending money to reduce headspace isn't going to improve accuracy much, if at all.
Here's a few "side affects". My bull barrel gun started out with the loose headspace bolt. About every 20 rounds +or-, I'd get a failure to extract .... In fact the barrel is stamped "WARNING: UNFIRED ROUNDS MAY NOT ALWAYS EXTRACT". With the tighter headspace bolt, the spent cases were flung farther and I never had a single failure to extract. I ended up leaving the tighter bolt in the bull barrel gun for that reason. The other 10/22 with the factory barrel never hiccuped once with either bolt so I'd say it is just as happy with the looser bolt.
In all, I fired about 20 more cartridges using the tight bolt than the loose bolt. When I did a field strip for cleaning, the tight bolt had way less powder residue than the loose bolt. Apparently, the tighter headspace helps prevent powder residue from blowing back into the action.
So despite the lack of improved accuracy, in my guns ... extraction was improved as was powder residue fouling when a tighter headspace bolt was used.
YMMV ... I'd be very interested to see other 10/22 results with headspace changes. Personally, I think someone is making money on a non-value added accuracy modification.