7.62mm ASHOT rifle - Part 2: Locking mechanism
Previous Chapter - Part 1: Introduction and visualizing the concept
This is the most challenging part of the whole system, way more difficult than I initially anticipated. The bottle neck cartridge causing the bolt thrust to be tremendous. Without carefully design the lock will fail and render the gun un-operatable or worse, cause serious injury to the user.
1st design
This locking mechanism directly copied from the Ashot shotgun, with 1/2 the long nut removed, along with separated barrel and chamber locking system.
But I quickly realize it was too cumbersome, too much welding and too thick. Also, more metal = more weight.
I still managed to made a POC out of it. But I quicky scrap this design for a more elegant one.
2nd design
Has a thinner profile, used 8mm bolts
8mm bolt threads are rather shallow. This cause the lock to easily slip, lead to a breech failure
Testing shows the locking nut isn't strong enough to withstand the pressure since there's too little surface area for the weld material to bite on to. Try beefing up the weld spot and there is not enough clearance for the bolt to drop into engagement
3rd design
I use a removable bolt (in green) as locking medium. Remove the bolt and that will allow the breech to pivot, opening the breech
But this is a failure. There's nothing to support underneath the locking bolt, thus it will strip and sheared
 |
| Notice the gap between breech and chamber. The lock slipped, causing the upper bolt to bent. Gun is cooked. |
Upgrade to 10mm bolts, hoping it should provide deep enough threads for the bolt to bite into.
Initial test was OK. The bolt hold up to pressure. No sign of failure (yet).
Testing was done under the water to reduce noise
But when I put the lock under stress test on a 50cm rifled barrel, with 200% over charge the lock failed.
No catastrophic explosion, just a bent hinge that took it out of commission. After some parts replacement it can be shot again, still a critical safety hazard. Not use worthy.
Close inspection shown that the bolt was sheared off from both sides
When the bolt was under too much stress, the thread broke off, the rod misaligned => runaway failure.
Broken thread from the bolt embedded into mild steel nut.
4th design
To give the thread proper support, I go back to the original ASHOT shotgun design.
The force from the cartridge only impart on the breech face, the threaded rods and the collar retaining the barrel. The weld spot on the chamber under little to no stress, which mean I can just duct tape it in place without any kind of welding, and the gun would still works!
Steel locking bolt has more strength than iron. This will prevent the shearing problem we saw earlier.
I don't want to remove the old nut and weld a new one, so I just use a hose clam to hold them together. All this are just prototypes and I just need them to work, no point making them pretty at this point
Since the weld spot on the chamber don't have to withstand pressure directly. Only a bit of tack weld is enough to held the chamber and the hinge together.
Also with minimal welding spot, the chamber will be less prone to metal warping. Reaming the bore afterward will be easier, and if your welding skill is good enough, the metal warping would be so little that reaming wouldn't even be necessary.
Test fire shows an OK result, with no crack or case head separation whatsoever
This "works" to some extend. But again, with 200% over pressure test on a 50cm rifled barrel, the lock struggle to keep up. This make me not fully comfortable with the safety level just yet. I gotta keep pushing.
5th design
This time, instead of the locking nut being welded permanently onto the chamber wall, it was made as a detachable part. Allow me to change the lock easily if I ever need to, without having to cut and weld everything all over again.
By rotating the bolt at the rear, the breech is secured and able to withstand the tremendous force created by the bottleneck cartridge.
This approach reduce the reloading speed, but safety has a higher priority in my list, so I'll stick with this for now.
The plastic shield is made of PET bottle and ductape, to protect the user from the hot gas escape from the breech face.
At the moment of this writing, I'm still not able to prevent gas leaking from the rear of the cartridge. And being too close to the shooters neck, this can still cause some damage
The shield redirect gas rearward and over the shooters shoulder. No more burning neck
This conclude this chapter.
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