How does roller-locked blowback, the mechanism used by MG 34s, G3s, et al., work? As I understand it, the bolt chambers a round and the firing pin pushes the rollers into barrel recesses to lock it, but I don’t understand how it gets unlocked again. Also, on a G3, what is the tube on top of the barrel for?
There’s a tube attached to the front of the bolt, on top. This goes into the tube on the top of the rifle, which holds the charging handle. Why is it there? Someone thought it was a good design. I used to have an HK-91 (sold it, bought an FN-FAL, sold it to get an apartment, and now I have a SAR-8 – which is the same as an HK-91) and I find it rather awkward. You have to flip it out and then pull back against the heavy recoil spring. Some people think that it’s a good design because the shooter’s hand is on the fore-end anyway. Makes it easy to reach. But I think it’s a pain.
Another thing is that there is no hold-open device so that the bolt stays back upon firing the last round, so you have to pull back the charging handle when you change out an empty magazine. There is a cut-out at the rear of the tube that allows the shooter to hold the bolt back. After inserting a fresh magazine the shooter need only slap the handle down. But I think a bolt catch mechanism activated by the magazine would be better.
Last year a friend (who had been in the army, but whose weapon was an M4) field stripped my SAR-8. He locked the bolt so the rollers came out and the bolt could not be re-inserted into the receiver. I did that once with my HK-91, and learned never to do that again. After so many years I’d forgotten how to unlock the rollers. It took me a few minutes to remember.
Specifically, the rearweard force pushes the bolt back, wich carries the rollers back with it. Due to the ramp shape of the locking channel on the barrel, the rollers are forced inward, shich pushes agains the incline of the locking sleeve, pushing it back. It takes a lot of force to do that, so the pressure must build up for a short time before the rollers are released.
I haven’t actually studied it, so this is going to be a WAG post.
When the bolt is in battery it is pushed back against the bolt carrier. (Or perhaps the bolt carrier is pushed forward against the bolt.) This locks the rollers into their detents. The Wiki article gives the pressure of the fired round as 50,000 lbf/in[sup]2[/sup]. That’s a lot of pressure. i think what happens is that the pressure is so great that the rollers push agains the bolt and, since the bolt can’t move, they push the bolt carrier backward, which unlocks the rollers and allows the assembly to move backward against the recoil spring. The small interval while the rollers are overcoming the spring pressure allows the pressure in the chamber to decline to a reasonable level.
So it’s the rollers that drive the locking piece out of the way? I always figured that the case head was somehow bearing on the locking piece, but then that made no sense because pulling the locking piece out would keep the rollers from delaying anything.