tilting block versus rotating bolt: advancement or just different?

[doofus47]

I notice that that most modern popular semi-auto rifles use the rotating bolt system.

I've had a number of older rifles: Hakim, vz58 and sks that used a tilting bolt action.

Is there some particular reason that rotating bolt seems to have displaced the tilting bolt?

[dakota.potts]

The rotating bolt is believed to have more repeatable lockup and better accuracy due to consistent and even pressure on the breech face. Whether this is true or not, I couldn't say. Some of the rifle systems known for better accuracy like the AR and M1A use a rotating bolt, and most of the popular rifle systems (SKS, VZ58, FAL, etc.) that use a tilting block lockup are not known for accuracy as great. I don't know of any tilting block systems where a model of firearm as a whole that uses it is known to be as accurate as one with a rotating bolt.

I would imagine that the tilting block/bolt system might be easier to machine but that more CNC technology has made rotating bolt systems more available for different action types. That's a complete guess on my part though.

[marine6680]

Tilting bolts cause uneven stress on the components. All the stresses go into one locking point. That is a disadvantage.

That uneven stress causes some odd barrel harmonics and forces during firing, and it can affect accuracy.

Tilting blocks are a bit easier to design and tune... converting linear motion into a torque is a bit harder.

BTW... A Vz 58 uses a locking block arrangement, its similar to something like a beretta 92, not a tilting block like a SKS. While function is similar on several ways, it is not the same. They are generally considered more accurate on average than an AK that Has a rotating bolt. But it has more to do with the overall design of the rifle than the locking mechanism.

[dakota.potts]

They are generally considered more accurate on average than an AK that is tilting block. But it has more to do with the overall design of the rifle than the locking mechanism.

Do AKs not use a rotating bolt system?

[Gunplummer]

He probably just meant to type SKS. I am not a believer in the accuracy not being enough for what it was designed for. I own a few and switching brands of ammo affects tilt block rifles more than the design of lock up. I would take an SKS over a turn bolt AK for accuracy any day. JMO

[marine6680]

I actually meant to type rotating bolt, it was meant to convey that it's more than just the type of action that leads to accuracy. I will edit and fix that.

And a tilting block is usually accurate enough for what they are intended for. Early designs worked well and were accurate enough, but the idea back then was marksmanship at extended ranges was what warfare would be about. Later, designs moved from that idea, and tilting block was used in a few rifles, the sks being a well know example.

[kozak6]

The biggest reason is cost.

Typically a tilting bolt locks directly into the receiver, which means the receiver has to be a large expensive piece of milled and tempered steel.

With a rotating bolt arrangement, only the trunnion or barrel extension have to be milled and tempered steel, and the rest of the receiver construction is inconsequential. It can be plastic, aluminum, or stamped steel. It only has to be able to withstand heat and recoil.

A rotating bolt also allows for camming action during primary extraction, which can offer an advantage in reliability.

[doofus47]

marine 6680

BTW... A Vz 58 uses a locking block arrangement, its similar to something like a beretta 92, not a tilting block like a SKS.

I stand corrected.


Thanks for the info! Just curious.

[44 AMP]

The biggest reason is cost.

Typically a tilting bolt locks directly into the receiver, which means the receiver has to be a large expensive piece of milled and tempered steel.

I would pretty much agree with this, bearing in mind that it applies both then, and now, in exactly opposite ways.

THEN, (and remember this was back in the days before CNC machining), ALL firearms receivers were large expensive pieces of milled and tempered steel.

(you see a change begin during WWII, first with SMG which don't have locking systems so a light weight receiver -stamped sheet steel- is actually practical)

Using a locking block /tilt bolt system was actually easier to make than a rotating bolt (means cheaper). Fewer and less complex cuts needed.

Today, the opposite is in effect. Modern machining means a multiple lug rotating bolt doesn't cost significantly more to make, and the entire receiver need not be strong tempered steel, only critical points, if any.

[TRX]

There's also the Degtyarev flap lock. Instead of the bolt tilting, "wings" stick out to lock it in place.

[Gunplummer]

I don't know who Degtyarev was, but the same design was in the M-85 .50's. What a complicated mess that was.

[44 AMP]

Bad as it was, the M85 was still a less complicated mess than the M73/M73A1/M219 coax.

[Gunplummer]

Yeah, kind of like disposable machine guns. I often wonder where they all ended up. You would think that they would pop up on news film in some out of the way country. You rarely even see the surplus places selling parts from them.

[44 AMP]

With things like US tanks and their guns, when they are "retired" from service, they go one of two places, mostly. They go to other places as foreign aid, or they go to the scrap yard as cut up bits and pieces. Mostly.