Last Post: 07/08/2010
Retrieved: 01/21/2011
Updated: 05/28/2016
Here is my latest build.
It is chambered in 5.45 and fed with an RPD top cover and Rpd "U" style links.
I have been calling it a XT74 which is short for extreme74.
I need to take some pics in the sun as my shop is kind of dark.
It has a drop out aluminum trigger box with AR15 trigger group. I made the trigger group very compact to keep the lines similar to the AK74.
I machined the receiver from 1/4" flat bar stock.
The carrier is an AK74 carrier heavily modified and custom gas piston.
Doubletapme
I like the way your FCG module goes up from the bottom. That would eliminate the outside grip stick and extra trigger on the ES version. I admire your workmanship and creativity in your builds. It is much better than mine.
I just got around to test firing it again yesterday. I can't believe it took a whole month to get around to it again. It ran about 96% but had several failures to eject. I will do some more fine tuning and try again. Its not done until it runs 100%.
I think I will make an aluminium heat sink sleeve like the japanese used in WW2 to help keep this small barrel from overheating.
It looks like you built a larger diameter gas tube coupled to a Tantal gas block. Did you have to do that to get enough shove to actual the feed cam with a smaller gas port because of the smaller bore than builders are useing on the various 7.62x39 MG47 version?
The gas tube I turned from a chunk of scrap. I did leave the side walls thicker and the receiver end was oversized and flats cut to slip tightly into the receiver. There is a tiny cap screw that locks the gas tube in position. The gas block is a Bulgy 90 degree with the lugs removed.
The 5.45 and 5.56 generate higher chamber pressures than the 7.62x39 and require a much smaller gas port. I actually chose to drill my gas port smaller than standard 74 because with the short gas system and full length barrel the piston receives a longer impulse duration and it worked out perfectly. I had to put a second spring in my buffer to keep the carrier from hitting the rear trunion.
Let me know if I didn't explain something completely. I am planning to tear it down again this week and will get pics of the gas tube removed from the weapon.
I see you moved the gas port back - did you plug weld the original port, or tap and screw a plug into place?
What is the velocity you are getting with this? I'm curious how significantly that changes the velocity since the gas is bleeding sooner.
Does that cause additional powder residue build-up in the gas system? I would think it does since the timing has changed...?
What modifications do the belt links did you need to do? I don't know what RPD "U" style links means? Do you mean "normal" RPD links, or are there different versions?
The way I plug a gas port is to drive a tapered plug into the port and rosette weld over with the TIG. This method ensures the plug wont move at the moment you apply heat and the plug prevents the weld from sagging and causing an obstruction in the barrel. I like to get on the heat fast and complete the weld in about two seconds to minimize the heat affected zone. I allow the weld to air cool as I believe quenching would cause uneven stresses and possibly warp or otherwise harm accuracy. This method has always worked for me.
I don't have a chronograph so I can't say exactly but I would expect it to be the same as any other Bulgy 16" chrome lined barrel of equal bore dimension. The amount of gas that flows into the cylinder is extremely small and has no signifigant effect on muzzle velocity. In a properly timed weapon the bullet leaves the bore and bore pressure rapidly drops before the piston head has traveled even the half inch length of the cylinder. The gas cylinder pressure drops a couple thousandths of a second AFTER the bore pressure. If this were not the case the breech would be unlocked before the chamber pressure dropped and bad things would result.
There are many many factors that effect the timing and we can discuss them later. The point is that the bolt carrier in this build is slightly heavier and the belt feed mechanism adds resistance both of which increase the kinetic energy required to accelerate the carrier mass. By locating the gas port closer to the chamber it increased the impulse duration on the piston and allowed a shorter gas piston which helped reduce weight of the carrier mass. So it kept the timing closer to where I wanted it.
I know some would say to drill a larger gas port or cut the recoil spring but those methods do have shortcomings we can discuss later. Its past my bedtime.
I hope no one thinks I'm trying to be a know it all as I have been misunderstood before. I'm just sharing my thoughts on designing custom builds of this sort.
Quote:
What modifications do the belt links did you need to do? I don't know what
RPD "U" style links means? Do you mean "normal" RPD links, or are there
different versions?
The RPD links do come in two different styles. One style uses friction to hold the case against the L shaped rear of the link. The other style has a staytent formed in it that engages the extractor groove on the case to retain it. They are usually referred to as L style and U style based on their shape. I used the U style and squeezed each link just a little to reduce the inside diameter. It works great. Just go easy when you squeeze them or you will snap the spring steel.
Quote:
I'm not sure would call that AK based?
The bolt, carrier, trunnion, barrel, gas system, etc. on these weapons are AK parts that have been modified to accommodate the RPD feed mechanism so I don?t really think it is incorrect to say it is based on an AK.
Quote:
Looks incredible. How many hours of work went into the build?
I just worked on it in my spare time so it is hard to say exactly. I would guess maybe 30 or 40 hours in it spread out over many weeks. Working on custom builds is play time for me and I just wish I had more time to play.