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Thread: .243 win based on AK innards

  1. #1
    Gunco Member kragaxe's Avatar
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    Default .243 win based on AK innards

    I drew up some plans for a rifle back in march and have been slowly collecting material and various parts as time/funds permit.




    I'm starting with a TAPCO flat and the front end off an M70AB2. Originally, I was going to custom make the entire trunnion, but I thin I can modify the milled stub off the M70 easily to avoid a complete scratch build on the front trunnion.

    I decided to go with G3 mags for the 243 shells. The reasoning behind this was the simple fact that I think they are cool as hell looking for a mag. The gas block/forward sight and muzzle attachement are also going to look a lot like a G3/HK, so they'll compliment each other.

    I'm still undecided on the BCG. I'll probably just modify the M70 BCG. I might have to redo the upper half of the carrier to fit my low-profile needs, but doing that and redoing the piston shouldn't be too hard. Due to a longer gas tube, i was already going to have to redo the piston.

    Forward grips will be made from 6061-T6 aluminum and freefloat around the barrel assembly. The barrel is a Rem mod-700 in 243 win. I cut it down to 17.5" and the muzzle is target crowned and outside threaded for 9/16"x24 (i already had those tap/die combo).

    I finished my AK bending jig and got it tweaked so it bends the flats to an inside width of 1.245". To use the G3 mags, I'm going with a magwell. I bent up one using 0.032", 1050 steel. This is nice in that it fits just inside the AK frame when properly sized to fit the mags and when the dimples have been flattened. In the pic, it's still a bit large. It'll get the top 1/2" or so trimmed off and fit up into the frame higher than what you see here. I won't trim it until I get the trunnion made and mounted and the rails welded in. It took three tries to get the "perfect" magwell....I don't want to trim too much and screw it up!

    I'll get to work on it next weekend...assuming my wife doesn't go into labor this week! ....even then, I still might get to work a bit.

    Here's pics of the magwell fitting:




  2. #2
    No Hope For Me 1biggun's Avatar
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    have at it. I assume you have seen the converting to a 308 thread? there a lot of good info. Id recomend reloading and keeping the pressures down 243 max loads might be pushing it. now one has blown one up yet but you donet want to be the first its a pretty tight fit to get a G3 mag to work it will not fit into a m70 trunion as Is. Are you planing to trim the mag were the lower trunion rivots are? Im interested in what you do for a Mag latch.

    Im going to be doing a 260 rem on a 308 sagia 3 lug bolt and trunion. it will be free floating barrel much like my other varmit builds. it will shoot full power loads. I toyed with 243 or even a 22-250 on a standard trunion I would be keeping the pressures down however.

  3. #3
    Gunco Member kragaxe's Avatar
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    I had originally planned on making my own trunnion and bolt. Looking at the milled stubs off the M70, I should be able to use those with no real problems. The whole trunnion assembly will be moved forward to accomodate the larger mag. The magwell section of the trunnion will get removed/opened up to fit the front of the custom magwell I made. The sides and bottom of the milled stub will be milled to fit the receiver flat. The original lower trunnion rivet hole will be one weld spot for the magwell. The other two rivet holes will be weld spots for the receiver flat to the newly milled trunnion. It may also get a rivet through it if I can find a spot that goes through the "meat" of the trunnion and doesn't interfere with the barrel.

    I'm hoping the only mod I have to do to the mags is to dremmel down the rear face to allow for the bolt to slide over and strip a round easier. The AK bolt looks to hang down a bit lower than the G3. The mag release will be put where the open area is at the rear of the magwell. This will be similar to an AR-15...just a lever and spring that "clicks" into the catch opening on the side of the G3 mag. Mags will insert straight up into the magwell, not rocked in.

    I plan on doing all reloads, although I can't see anywhere that 243 pressures would cause issues, maxing out at 60k. I'll probably be in the low to mid 50k with the loads/bullets I'm looking at. The lugs look plenty thick enough compared to several other 243 bolts I've looked at on bolt actions. Although that depends on jetal types and heat treat and such. I'm using a 1" dia chamber section on the remington barrel, plus the trunnion thickness. I think the gas adjustment may be the pain on this one. I'll start with a small hole and work up until I get proper function.

    Hopefully Saturday I'll get more work done on it.

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    Gunco Regular Gunter's Avatar
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    If you a using a standard 7.62x39 bolt and carrier, then if you move the front trunnion foward (about 3/8 of an inch I assume) the rear of the bolt will be too far foward. Which would allow the hammer to hit the center support instead of the firing pin.

    I think, if I can remember right, so you might want to check it out.
    "Take time to deliberate, but when the time for action has arrived, stop thinking and go in."

    "Four hostile newspapers are more to be feared than a thousand bayonets."

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    Gunco Member kragaxe's Avatar
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    Yeah, that was one reason I had planned on making my own bolt. I wasn't sure if I could just make a new hammer with a little more reach, or if I would need something more drastic like a whole new bolt. Witha new bolt, I'd just add the extra length to the end and make a new firing pin as well. I'd have to account for the extra boltlength when the BCG is in the fully rcoiled position. The recoil spring mount and rear trunnion would have to be customized for that, but that would be pretty easy. Support pins can always be relocated, so that doesn't concern me at all.

    I get my springs and such in hopefully tomorrow. Then I can assemble the FCG and see what all I can do with a different hammer shape. The distance is more like a 1/2", so it's a pretty big alteration. The hammer would be longer with a slightly different angle on the upper end. The extention doesn't look like it will affect cocking/locking and there's plenty of overhead clearance, so it'll just be a bit of trial and error. The reduced spring tension with the extra rotation might be an issue, though. I'll be sure and take plenty of pics of all the mods as I do them.

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    Happy Camper hcpookie's Avatar
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    This will be just like the .308 builds except of course for the barrel. You'll need to relocate the ejector.

    You'll also have to move the center support back for the mag clearance. You may also have to enlarge the lower rivet area of the trunion for the bigger magazine. A FAL mag would provide better clearance and let you keep more of the meat on the trunion. Also would have a BHO feature similar to PSL

    You can keep the FCG where it is but you'll have to reshape the front of the hammer to function properly with the center support being closer to it now.


    Wasn't someone else working on a .243 build?
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  7. #7
    Gunco Regular Gunter's Avatar
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    I was thinking that Aloharover was a 243, but it was a 308.
    http://www.gunco.net/forums/f244/alo...-hybrid-24686/

    Well in theory, if the front trunnion is moved forward, then the ejector might be fine in the stock postion. However the bolt/firing pin would need to be extended and have the ejector slot lenghtened in the bolt so that it would close and rotate into the locked postion. If you add material to the face of the hammer to reach the standard bolt, then you might have problems with clearence between the hammer and carrier when cycling.

    On the other hand, if the trunnion stays in the standard place, then the ejector would need to be moved rearward and again the slot in the bolt lenghtened for the new ejector placement. Also moving the center support back to clear the mags can cause problems in that the hammer spring hits the support and keeps the hammer from being installed (learned that one the hard way).

    Both ways have their pros and cons.
    "Take time to deliberate, but when the time for action has arrived, stop thinking and go in."

    "Four hostile newspapers are more to be feared than a thousand bayonets."

    Napoleon Bonaparte

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    Gunco Member kragaxe's Avatar
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    Instead of cutting rearward for mag clearance, I cut forward and chose to move the trunnion mounting location. Moving backwards looked like it caused more problems than fixed. The rear of the mag is in the exact same place as the AK mag, so the support pin won't need to be moved due to mag. Likewise, the ejector "should" work in it's present location. Due to a larger diameter shell base, I might could trim it a hair, but that'll be one of the last things I worry about. I didn't think about the bplt rotation with the ejector in a different position...thanks for pointing that out!

    Hammer mods can be one of two things......Thicken the hammer face to make up gap or lengthen hammer and allow for more rotation to allow proper strike. For a 1/2" gap, thickening the hammer face is out of the question. Instead, I'll lengthen the hammer and adjust the angle of the face to make up for the extra rotation needed for it to make contact with the firing pin. Unless, of course, I opt for building a new bolt.

    I'll have to widen the cut out in the trunnion where the mag fits in. That's pretty easy to do. The meat on the walls of the trunnion stub in that spot don't concern me. I don't plan on riveting there. I was considering drilling a few holes or a slot, then backfilling with weld for the trunnion/receiver connection. There's a few locations I can stick a rivet as well. Once I get the trunnion stub fitted/located, I'll figure out exactly what I'll need for mounting.

    Of course, once I've finished a couple months of planning and building I'll be heat treating the entire receiver, so if anything is going to screw up the build, it'll probably be that! Hah! I have a jig I'm making that should work pretty well for holding shape, though. It's basically a tapered I-beam That will insert into the receiver and screw onto the receiver. The custom rear trunnion will be on it as well during HT so that will help as well. All part of the fun, right!

  9. #9
    No Hope For Me 1biggun's Avatar
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    I plan on doing all reloads, although I can't see anywhere that 243 pressures would cause issues, maxing out at 60k. I'll probably be in the low to mid 50k with the loads/bullets I'm looking at. The lugs look plenty thick enough compared to several other 243 bolts I've looked at on bolt actions. Although that depends on jetal types and heat treat and such. I'm using a 1" dia chamber section on the remington barrel, plus the trunnion thickness. I think the gas adjustment may be the pain on this one. I'll start with a small hole and work up until I get proper function.
    Be very aware that 2 lug fail the pressure tests required to allow there sale to the puplic in europe and likely here in the USA. that is why the the 308 Saiga has the third lug so it passes those proof tests. 60,000 PSI is a hell of a lot more pressure than the standard 7.62 round of around 43,000 PSI. also you need to factor in the differance in bolt thrust as the .243 round has a bigger case Bolt thrust is what you need t worry about that weaht the lugs see. I dont now what 243 you looked at that had simular sised lugs but every bolt action I have it much bigger. rem 700, win 70, mausers, rugger, savage are all much bigger. and have much more engagment area.

    welding to the trunion is a really bad idea especialy on the upper area at that is were the lugs are. Any welding regardless how small it is is a stress riser I dont care if its the size of a bin head a weld affect the metal that it touches and the area directly nect to it most cracks start at a very very small inclusion or stress riser. I would not risk it especialy t the increased pressures as well as the recoil is greater meaning more pressure n rivets , screws or welds depending how you attaach the reciver. The m70 trunion from a stamped trunion has much more material than the Stubs from a milled kit. the trunion is not designed to add any strength to the barrel its there to hold the barrel and provide lugs for the bolt to push against.

    This will be just like the .308 builds except of course for the barrel. You'll need to relocate the ejector.

    You'll also have to move the center support back for the mag clearance. You may also have to enlarge the lower rivet area of the trunion for the bigger magazine. A FAL mag would provide better clearance and let you keep more of the meat on the trunion. Also would have a BHO feature similar to PSL

    You can keep the FCG where it is but you'll have to reshape the front of the hammer to function properly with the center support being closer to it now.


    Wasn't someone else working on a .243 build?

    like he said. no reall differances from a 308 build .

    you dont need to do all the mods your proposing simply move the ejector back. move the center support back cut the hammer to clear the new center support location, enlarge the bolt face, open up the ejection port on the dust cover, use a M70 bulged milled trunion for a little extra protection.

    keep the loads at a lower pressure. 60,000 PSI will likely get you in trouble or dead. there are many MAX 243 loads at only 50,000PSI with certain powders, you should be able to find a good strong load in the 46,000 PSI range. i cant reach max loads with out pressure signs any way so its likely you can get a ggod load to suit your needs. what is the intended purpose for this build? what weight bullets do you intend to use.

    the 243 is often facoty loaded hotter than a 308 because there are no old military weaker guns chamberd in it so the factorys dont have to worry as much.

    the mag well you are using will work but it dont need to have it extnend into the trunion just stop it at the reciver bottom. if you trim the front of the mag it will fit you will need to rework the follower stop. the demples in the mag guide the round so the last 3/4" dont do any thing so that area can be lowered. I had one built up that would use a G3 but I ended up going with a FAL mag and never completed it with the G3 set up. the gs are cheap but they also are harder to get to feed right. I dont see any real differance in the clearance between a AK mag and a G3 as far as the bolt hitting it. in fact its nearly identical to a 223 in the clearance and just slightly shallower than a 7.62 round. a bulgy 223 bolt is the same as a normal x39 bolt so I see no need to cut the rear of the mag. it should catch the round no proplem. moving the magwell back takes up some finger clearance in the guard but if you use your style of mag well and ditch the stock AK style catch you gain most of it back as you will loose the are were the catch and the spring go.

  10. #10
    Gunco Member kragaxe's Avatar
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    I just read through the Aloha thread. I hadn't seen that one yet...what a cool build!

    "Do you mean turn some threads on the back of the bolt, and then make up an extension, also threaded and just screw the extension onto the bolt?

    That is a pretty cool idea."

    I pulled the above from it. Hah....what an unbeleivably easy fix! Sometimes the simple things just get missed! Custom rear trunnion and mod to the ejector slot.

    Man...now I feel like playing hookie from work to do some gunsmithing. Of course, my wife will be going into labor sometime in the next couple weeks. if I get the grandparents to watch the kids and change diapers...I can work on guns, right!?

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