notebook: bolt lugs

TRX
September 20, 2013
Breech loading firearms cropped up a few times in ancient history, but all modern "turnbolt" designs descend from Nikolaus von Dreyse's "needle gun" of 1836. The needle gun, so named for its long firing pin, used consumable caseless ammunition with front ignition and a saboted lead bullet instead of a round ball; it's worth looking the needle gun up when you have a free moment.

Anyway, the Dreyse used the bolt handle as a single locking lug. Hiram Berdan's No.1 and No.2 rifles also did that, and the 1869 and 1871 Mausers, and various bolt-action shotguns, right down to the current day. The Krag rifle used a single front locking lug with the bolt handle as a safety lug, but that was a fairly unusual variant.

The German 1888 "Commission Rifle" used a forward, two-lug bolt of what we now think of as conventional design, other than the separate non-rotating bolt head, which I personally think was a great idea. It probably wasn't the first forward-two-lug design, but I'll use it as an example.

Right about that time James Paris Lee came up with his two-lug, rear-locking design, most familiar simply as the "Enfield" or SMLE. Though it became the standard battle rifle of the British Empire, it was, developmentally, a dead end, as most modern bolt action rifles descend from, more or less, the 1891 Mauser.

Now, this sort of generic bolt action looks quite simple. But it's not simple at all. For the Mauser design, you need long raceways cut into the receiver. We've discussed that in another thread. Some designs turned the locking lugs 90 degrees, so the bolt lugs were vertical instead of horizontal when the bolt was worked; the Mosin-Nagant is a common example of this. Vertical lugs also kept the extractor and ejector cuts out of the locking lugs, and the bottom lug helped strip cartridges from the magazine, making feed more reliable. But the cost was a deeper magazine, as the lower lug swept through the space where the top cartridge ordinarily went.

Okay, that's all simple enough. We'll continue with the Mauser as an example. We have a horizontal bolt head and a receiver with long raceways. Run the bolt down, turn, and if everything is clean and headspace is correct, the bolt should turn down and lock. But we can't guarantee that, particularly in a military rifle. A bit of dirt in there, and if the bolt can't go all the way forward, it won't turn. So we round the forward edges of the locking lugs to help them cam the bolt into place against resistance. In fact, we cut cam angles on the lugs and matching cam angles on the receiver, and cutting those can be a real beeyotch on some designs. Now when we turn the bolt handle down we have cams to handle balky ammo.

Unfortunately, that comes at a cost. The cams come out of the locking lug area, which means the bolt and receiver have to be harder to avoid "bolt setback", which is when the bolt lugs dent the receiver. Also, if we go to a three lug bolt to try to reduce the amount of bolt lift, the cams still have to be the same length, which means we lose overall area. That's why many three lug designs wind up as six lugs, in two rows of three; to get more face area to prevent bolt setback, while maintaining good camming action.

Ah, and we want to be able to extract balky ammunition, so we need cams to pull the bolt back, too. Mauser-style rifles put the cam on the "bridge", the ring at the back of the receiver, and the bolt handle moves against the shaped bridge. Mosins and others put the extraction cam on an angle cut at the back of the receiver, up front, and have a side lug on the bolt to engage it.

There are other tweaks. The 1905 Ross, for example, had the locking lugs cut at a substantial angle, so the entire lug formed the closing cam. The main reason, though, was that the Ross was a straight pull design, and the angle- cut lugs release immediately as soon as the bolt starts to move, making the bolt easier to open, since the Ross had no extraction cams. The Pattern 14 and 17 Enfields look like conventional Mauser variants, but they had their lugs cut on an angle as well, though much shallower than the Ross, and used Mauser-style cam cuts too.

Somewhere around 1900 came the multilug bolts. The early Newton rifles used a coarse thread on the front of the bolt, then cut the sides away. The bolt head looked like the barrel threads of some takedown rifles. The threads were easy to cut on the bolt and the receiver and the cam angles were built in. The 1910 Ross design looked similar, but as far as I can tell it was two rows of shallow angled lugs instead of an interrupted thread. Same basic deal, though.

Things pretty well stopped there for a long time. Then the "fat bolt" designs started showing up. I haven't found any examples dating before the 1950s, though I'm sure some exist. At this time "fat bolt" designs are found mostly on some limited-production hunting and target rifles. The fat bolt has the bolt body the same size as, or larger than, the diameter of the locking lugs. Both two and multiple lug designs are found, though the two lug variants seem to be confined to single-shot target rifles. The main advantage of the fat bolt is that it is easier to make than one with raceways. But like the vertical bolt lug designs, the fat bolt means either a deeper belly on the stock or the loss of a cartridge due to the diameter of the bolt.

Most rifles still have two opposed locking lugs. A few have three, claiming shorter bolt lift and a more stable "tripod" to hold case head thrust for the benchrest guys. I found one bolt action with four lugs, but apparently it didn't work too well and went out of production. The closing and opening cams had to be shortened to make it work, and bolt operation was stiff, supposedly.

When you get into autoloaders you can find six lugs, primarily in the AR designs, though there are others. We don't need no steenkeeng cams when we have a spring and bolt carrier to slide-hammer the cartridge in, and we have gas and the bolt carrier to slide-hammer it out.

TRX
October 10, 2013
All Ross rifles were straight pull. The 1905 had two opposed locking lugs, running vertically, like a Mosin. The bolt and receiver engagement areas were cut at an angle, turning the whole area into a ramp, but its main purpose was quick disengagement for the straight pull bit. Since you only have the "pull" motion on a straight pull, less force is available for primary extraction.

The 1910 Ross had a multilug bolt. On early models, the bolt could be disassembled (which wasn't recommended to be done in the field), then assembled wrong, and the bolt could then unlock when the rifle was fired, with serious consequences for the shooter. Ross recalled them all and modified them so the bolts only went together one way, but they had acquired such a bad reputation in the media that Ross got out of the gun business. Which is unfortunate, since the Rosses were innovative and well-made.