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I've been told that in order to make a good dmr, you should have an elongated gearbox, but I'm not sure why? What does it do?
How does an elongated gearbox help?
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Its all in the air volume.With the long cylinder,you have more air volume at your disposal to drive heavier ammo further down range,depending on what spring you use.There typically a friendlier GB to make this way,because you don't have to use as heavy a spring to get power as you would with a conventional length cylinder.With heavier springs(M140+),they start to become very powerful rifles(500fps+).
You don't need an SR-25,AR-10,PSG-1,L85,or even a T65K2(all of which are examples of long cylinder GB rifles).Its easier to achieve power,good power with them.I have built V2 GB's for rifles running standard,and boreup cylinders in excess of 500fps.When building a GB for a DMR,you have to be very anal about compression,and air leaks.If you can control these it will be much easier to achieve consistency from shot to shot,and if you can do this,accuracy will go up as well.DMR's are all about accuracy and consistency.
You don't need an SR-25,AR-10,PSG-1,L85,or even a T65K2(all of which are examples of long cylinder GB rifles).Its easier to achieve power,good power with them.I have built V2 GB's for rifles running standard,and boreup cylinders in excess of 500fps.When building a GB for a DMR,you have to be very anal about compression,and air leaks.If you can control these it will be much easier to achieve consistency from shot to shot,and if you can do this,accuracy will go up as well.DMR's are all about accuracy and consistency.
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92 Posts
Now I'm more of bolt action guy, so gearbox compression is not somthing I know too much about, how would you test for air leaks?
Simple,after you have assembled the cylinderhead to the cylinder,take a piston you know will have good seal and force it into the new assembly with a finger over the air nozzle stem.If the piston stops or if you let go of it and it bounces back its good.teheelmao1 said:
The next test involves putting the cylinder assembly in the gun with tappet plate and air nozzle installed.The tappet spring should also be installed to pull the tappet plate to its furthest forward position.Do the compression test again,if you acheive the same results your good to go as far as the GB compression goes.There is but one more weak link,and thats the air nozzle to hopup union.You want to make absolutely certain the bucking is on as perfectly as you can and sealed some way.Some use pipe tape,some whip with floss.I use Permatex Anaerobic gasket maker to secure my bucking to the inner barrel.On some high power buildfs ive done this and whipped with floss.The only other place you can loose air is where the air nozzle goes into the hopup.With half your GB installed in the upper receiver,hopup installed with barrel,and flash hider/muzzle break removed compression test the cylinder again,this time with your finger over the end of the inner barrel if possible.Bend an ear to the hopup.If it leaks you should be able to hear it.
That would be an inferior way to check compression.At best you could test the airnozzle for leaks as it would be extended as far forward as it can go.The piston would be in its furthest forward position,thus covering the airnozzle stem from the inside of the cylinder.Far better to actually employ the parts as there intended to move than resorting to static testing.headline said:
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