Long story short Im working on a LRBHO for a Saiga rifle. Not AR related, I know. But certainly technical discussion. Mods, move as you see fit.
Ill spare you the technical details of the home-brewed design until I’ve extensively tested it under unforgiving conditions and proven it worthy of sharing.
For now I need to know what the best method of attachment for a D2 bracket I made is. It sits just in front of the hammer in the receiver. Either attachment type would be used 4 times, once on the left side of the receiver, once on the right and twice on the bottom. They will all feature long axes perpendicular to the bore and all will indirectly bear the majority of the force imparted by the BCG when locking and locked back.
Optional attachment methods: I know I can rivet it inside the receiver or drill, tap and bolt it in (with threadlocking compound and possibly a cotter pin, naturally) or refabricate it out of 10 series steel and TIG it in. Im opposed to 10 series for obvious durability reasons compared to D2 in this application. So is there an engineering-savvy dude here who can say definitively whether a 3/16" bolt or rivet would be significantly stronger than the other in shear strength?
If you look up the specifications for hardware, in particular aviation hardware, you should be able to find the shear and tension strength of different fasteners, such as NAS bolts, different alloy rivits and so on. I believe the document you are looking of is something along the lines of the Aviation Technician’s Handbook
Thanks for the Rosette weld suggestion, I hadnt even considered that. I would love to weld it in place to avoid any possible issues with the bolts backing out and to decrease the likelihood of the bracket eventually shearing off but I just really dont want to work with easily weldable steels.
Thanks for the pointer on the Aviation Tech Manual. Didnt find any shear strength ratings for their bolts but bolts do offer the ability to test function before a more permanent attachment… They seem like the best option for that reason. Only time will tell if I can trust the guts of my gun to Red Loctite.
And if I completely ruin the gun… Egh. Not a big deal in my opinion. Saigas aint exactly a work of art and they aint exactly expensive.
I was thinking maybe you could get away with still using the D2 tool steel while also still welding it…
drill holes through the bracket and the receiver, then slide bolts through the holes, mark the bolt where it flush with the receiver (just run a sharpie line across the bolt where it protrudes from the receiver), then back out the bolts, and cut them just a little after the mark, as in cut them just a little short so they don’t fully exit the receiver, then just rosette weld the hole, molding the bolt and receiver together while leaving the D2 bracket untouched…
this way you could test the function with bolts and nuts first, and then perm attach it once its all worked out by welding it flush and grinding it for a clean appearance
essentially just welding the receiver and bolts together with the D2 bracket sandwiched in between…
You may want to consider using the existing pins and rivets in the receiver to geometrically constrain the bracket rather than welding or punching more holes. You could drift out one of the FCG pins, slide bracket in and anchor it under one of the existing pins and lock it down with the FCG pin. You would have to design the bracket such that it is constrained by your chosen set of pins and that it distributes the impulse load of the bolt being caught such that it doesn’t damage the receiver (elongating holes and such).
You may find though that the sheet metal receiver isn’t up to the impulse created when the bolt is caught by the bolt catch (what was it, the HK G41?). Particularly if the bolt catch acts upon the bolt itself, the bolt will attempt rotate abruptly as if going onto battery, and will only be constrained by the guide rail. You may find that abrupt impulse, which is substantially greater than encountered when simply stripping a round off the magazine, will eventually damage/bend the bolt guide with extended use. This could be avoided if you choose to catch the bolt carrier rather than the bolt. Just stuff to consider.
Reagans, you are a genius. Thank you. Ill look into some weldable bolts. More streamlined, too. No bolt heads to chop up your hand when charging the gun.
JPB, good thinking with anchoring it using existing pins. Ill see how much space I have to work with and try to do this in addition to the welded bolts. The BHO is designed to engage only the carrier on its right side, it should NOT even touch the bolt. I plan to use an existing ledge on the carrier as the BHO contact surface. However, if this proves unstable I will have to slightly modify the carrier and completely refab the BHO with a larger contact surface. This design will also hold the carrier a little farther rearward in the receiver than one that simply engaged the bolt face, which of course helps when chambering.
Sounds like you’ve though much of this through. My perspective is from a standpoint of marketability. Not sure if that is your end goal or not. Being able to drop this thing into any AK with no permanent mods to the weapon itself would add to its appeal. The complete kit would probably constitute a bolt carrier, bracket/BHO widget, and magazine followers.
Ive considered that. No permanent mods and a drop in ability would certainly up the marketability. At this time I have little interest in starting up a business. Not only do I have little time for the many other projects Ive got but Im also shipping in no more than 10 months so… dont really want to start a company only to have to shut it down a few months from now. This is just an experiment with the AK platform, see what its capable of in terms of efficiency of its controls.
The AK bolt will not rotate when caught by a catch or while stripping rounds from the mag.
There is a flat spot in the cam track which prevents the carrier from applying any rotational force to the bolt UNTIL after the bolt has been partially rotated by contact with the barrel trunion.
This is an intentional design feature that reduces friction binding and increases reliability.
Any method of attachment will put a load on the receiver.
I thought the concern was the method of attachment, the front trunnion rivets are designed to withstand the exact type of stress the bolt stop will have.
A very good point, Todd. Do you think Id really notice the difference if I used bolts with beveled heads and filed out some beveled holes for them in the receiver before welding them in? I expect to never sell this rifle, so its expected service life can be estimated to be in the tens of thousands of rounds.
You want to take the load off the fastener. If you bolt it together you want the friction of the two parts pressed together to hold it in place. If you weld the bolt you can’t tighten it so the load is all held by the bolt and your weld.
I think I understand. I do understand that a beveled rivet would feature increased shear strength and possibly a longer service life, but as we’ve discussed, I cant adjust the fit of the bracket in the receiver if I rivet it in. I think Ill have to go with highly torqued and Red Loctited bolts. Now I at least know I ought to use bolts with beveled heads, though. Might distribute that load a bit better.
Or you could always put a bushing (think cylindrical spool) inside the bracket and run the bolt through the receiver the bushing and out the other side of the receiver. That would enable you to clamp the bracket with the receiver walls facilitating the friction fit necessary to hold it there long term without loading the bolt in shear.