Posted: 4/17/2013 9:59:23 PM EDT
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I'm now to the "fab up the bolt" phase of my home-build 22LR conversion for an AK project.
Looking at just a few guns I have handy, I have the following weights (these are all dressed bolts incl. extractors, FP, and FP springs, but without charging handles): Charter AR7: 157.5 grams. Marlin Model 60: 151.4 grams. Ruger 10/22: 185 grams Ciener 22LR AR Conv. 142.8 grams German 22LR AK Trainer: 184 grams I'd be interested in anyone else who can provide some additional weights so I can try to shoot for a sweet spot on this project. The other alternative I have in mind is trying the two extremes -- the light Ciener weight on the one end, and the heavy 10/22 weight on the other. In particular, do any of you WASR22 owners mind weighing your bolt for me? |
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Quoted:
I have a WASR-22. I'll weigh the bolt tomorrow if nobody does it before then. As (if not more) important than bolt weight will be recoil spring (resistance) weight. Its true spring weight will impact cycling quite a bit (the spring has to be light enough to let the bolt do its complete cycle, and yet strong enough to feed rounds off a FULL magazine. But in truth, recoil spring weight is easily fudged across a wide range. But the bolt weight cannot be easily altered. And its bolt mass, and only bolt mass, that keeps the breech shut until the pressure goes down. |
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The math says that the bolt should weigh 181.4 grams.
Bolt mass is THE most important thing in a blowback firearm, spring pressure almost doesn't matter as long as its strong enough to strip a round out and chamber it. for a 22LR the bolt thrust is about 950 pounds of force, the mass of the bolt needs to absorb that, springs wont do crap. here is an excel sheet containing specs for bolt mass for tons of calibers. http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CDYQFjAB&url=http%3A%2F%2Fwww.orions-hammer.com%2Fblowback%2Fcartridge_data.xls&ei=m5BvUfuxJ4XmiwK3tYGgBQ&usg=AFQjCNHUKHa3igPQH6kYXjaRfkyiF2VC-w&bvm=bv.45373924,d.cGE |
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Quoted:
The math says that the bolt should weigh 181.4 grams. Bolt mass is THE most important thing in a blowback firearm, spring pressure almost doesn't matter as long as its strong enough to strip a round out and chamber it. for a 22LR the bolt thrust is about 950 pounds of force, the mass of the bolt needs to absorb that, springs wont do crap. here is an excel sheet containing specs for bolt mass for tons of calibers. http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CDYQFjAB&url=http%3A%2F%2Fwww.orions-hammer.com%2Fblowback%2Fcartridge_data.xls&ei=m5BvUfuxJ4XmiwK3tYGgBQ&usg=AFQjCNHUKHa3igPQH6kYXjaRfkyiF2VC-w&bvm=bv.45373924,d.cGE No question, bolt mass does it all, at least in terms of keeping the bolt shut until pressures go down. Recoil spring is just to return the bolt to battery carrying a new cartridge. But notice here that some of these guys have some interesting shit going on. Ciener's bolt weighs WAY down on the low end. I think it must be for reliable cycling and making sure the bolt absorbs enough energy to cock the AR15 hammer. The argument would go something like this: In a "right" weight bolt, much of that 1000 pounds is absorbed/lost before the at-rest bolt starts to move. The speed to which the bolt accelerates in the short distance and short time before the projectile leaves the barrel yields the total energy the bolt can then carry into its "jobs" for the rest of the cycle -- extraction, ejection, recocking the hammer.. Since the force impulse is so short, the lighter end of spectrum of bolt weights allows that force to be translated into movement energy more readily before the force stops due to the bullet leaving the barrel, yeild a slightly lighter bolt moving with more gusto and able to carry that energy through its cycle. Its the difference between putting a baseball on a T and putting a bowling ball on a T and hitting both of them with a bat applying exactly the same strength. Of course the cost of the lower end of the mass range is that you risk that bolt moving too far back before the pressure has gone down. Which is probably why the Ciener conversions seem more prone to blown cartridge bases, and are less tolerant in that regard to dirty conditions that cause the bolt to start of slightly out of battery. My design is going to be ramped in back to as to push the AK hammer from the very top and to translate to a ramped situation that should give the best possible leverage in cocking the hammer, but given how heavy AK hammer springs are, I think I will start in the 145 gram range and possibly go even lower toward the Ciener weight if need be by drilling lightening holes if I don't get reliable cycling. |
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You could apply some math to the problem...
Force = Mass x Acceleration Kenetic energy = 1/2 Mass x Velocity ^2 Using the first equation you can determine the difference in the acceleration between the different bolts. (Being a linear relationship the difference isn't going to be much.) Using the second equation you can solve for the amount of energy the bolt has once it's moving and determine the amount of energy you must bleed (or have available) by recocking the hammer and compressing the recoil spring. You will want to consider the hammer weight of the various firearms as well since that will add on to the weight of the bolt when it is in the fired position. |
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Quoted:
You will want to consider the hammer weight of the various firearms as well since that will add on to the weight of the bolt when it is in the fired position. Indeed. And the AK (and AR) hammers are heavy -- plus the mechanical advantage of the hammer springs seriously impact the moment of inertia in terms of moving them in their rotational plane. As I said, my drawings have the top of the bolt acting on the top of the hammer, and then transitioning to a very "ramped" configuration as the hammer rotates back. That should minimize the "effective weight" added by the hammer to the system.... I think I'm going to shoot for the lower end of the weight range. |
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I would be very very careful about how lite you go, matter of fact if it was my build I would go with the weight the math calls for and lighten it if needed to make it work.
the potential downside to going too lite is that the bolt decides it doesn't like the gun and exits the rear of it. Bolt speed is nothing to be taken lightly. |