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RPD DSA receiver /Vector Arms FCG build how to

11K views 36 replies 15 participants last post by  Coils 
#1 · (Edited)
Since there is a dearth (hey 4thIDvet this word was for you) of information on this build and I have gotten numerous e mails from folk expressing the same concerns, I will attempt to document my build. I do not know if I am doing this right but between all of us I think we can figure it out.

This build will be on a Polish nonreciprocating kit sold by SARCO, a DSA receiver, and the FCG sold by Military Gun Supply (MGS) which comes with a set of extremely vague instructions that has Vectors name on them.

I chose to remove the gas block and sight from my kit as i will be using a borrowed lathe (his is nicer then mine) and the owner would probably not want me (a novice) in his shop swinging the eccentric and entanglement hazard mass. You dont have too and the instructions indicate leaving them on.

I used a chop saw to remove the bulk of material from the front stub to free the barrel. Then I used an air powered hand cut off saw (3" disk size) to cut the last little bit so I would not hack into the barrel by accident. I then used a hammer to knock the two halves apart. This took all of 5 minutes.

(pic of barrel)








Next I moved to the lower receiver. I removed the sear, safety, trigger and leaf spring per the included instructions. To remove the stock, remove the little cross bolt and then GENTLY tap out the stock with a soft covered mallet. The recoil spring that has been captive will no longer be restrained.

Place the lower in a vise and mill the 2 rear areas shown.

(pic of milling)


you want to leave the "middle finger" sticking up as it appears a FCG spring pushes against it.

(pic of sear in place)

Then using a 1/8" end mill clean out the front pocket in the trigger area. this is indicated on the plans.

If you try to put the lower on the upper receiver you will not there is about a .5" gap. This is one of those BATFE things so be sure to mill the LOWER to fit.

You will need to take about .080" off the right side and .241" off the left, unfortunately i lost one of my matching numbers on the lower to this operation. I also had to radius the back lower edge of the DSA receiver to get a tight fit.

(pic of gap)

(pic of lower rails notched)

Then flip the lower up on its left side (right side is up) and clamp. Locate the scenter of the safety hole and then using the 1/8" CENTERCUTTING end mill relieve the areas for the FCG pins. Then drill each hole .118" all the way through both sides.

(Pic of clamped lower)





BOLT CARRIER: After studying the drawings, stubs and receiver it appears the TOP of the carrier rail needs to be shaved. I know several people have commented the lower does but I just cant see it ass being correct with this particular set up.

RECEIVER BARREL WELL PREP:

I used an OLD smooth flappy to remove the heat treat scale. I would NOT use a new one as they can eat away metal pretty fast. Just go light here and it only takes a few seconds (literally) to clean the barrel pocket




BARREL TURNING

Ok I chucked it up with the muzzle end through the jaws and used an old go-no go gauge to center the live end. I expected some damage to the go gauge but there was none. Perhaps this is a way for you guys to turn the barrel with out having to make an insert. Ok I spun the threads off then measured the smooth barrel where it will mate into the receiver. .9435" OD The receiver measures .9410 to .9420" depending on where it is measured. I LIGHTLY touched it with the lathe. Then chamfered (spelling?) the leading edge to a 45 degree bevel and eased the first 1/8" or so to .941" to help get me going. I also made sure I used a file to smooth where the old pin went so I would not have an issue there during pressing. I was concerned with how thin the receiver is where the carrier slides underneath so I took the barrel to .9425 to .9230"




Lubed it and the receiver with black oil and pressed it home with a 40 ton jack. Went smooth as silk.


I did not drill my barrel pin for 2 reasons:
1. I have miss placed my bolt
2. I want to be sure all is well and straight before installing.

UPDATE 3-22-2011

Safety modification:

Lay down a TIG bead as shown in the instructions

I will grind this down tomorrow to fit the lower


Weld in the Carrier cross pin:




UPDATE 3-23-2011



Put it in the mill and remove .018" from the TOP of the rails.

Then remove ALL material 3.7" from the piston side down .018" ***this is NOT the dimension on the plans, this is what fit my rails****


Heres a better idea of what to shoot for.
all the shiny was removed.

 
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#8 ·
When you go to drill the FP extension, you would enter the hole the cross pin is in at the edge, the bit would walk. so the cross pin is there to make the carrier "solid" if it was not welded in it would turn as you drill.
 
#11 ·
It is already pressed in (so pics and measurements are difficult)but the cut is 12mm wide and it is curved.



It looks like a concave shell mill was used to notch it.

Looking at the picture, think of moving right to left across the extractor cut, as you start moving it bulges out towards you then recedes back.

Looking closer I see that is is curved to make sure the case head is supported. If it was cut with a square end mill the curve of the chamber would cause a "v" across the extractor notch and thus an unsupported case.
 
#14 ·
Thank You VZ.

I have a US made barrel for my RPD kit that has no extractor relief cut. Looks like it will be tricky to recreate.
Pirate has a tool for this, he has a pic of it in his section on here, see if he can just do the extractor cut for you. He is busy with builds right now though.
 
#13 ·
you might have to go with several 1/8" cuts , rotating the barrel after every cut. What I would do would be square end mill it until the center is close to the rear of the chamber. Then 1/8 either side then file and dremel.
The whole bottom edge at the chamber is almost knife edge thin.
 
#17 ·
Pirate had a build sequence that showed the amount of material had had to come off the bottom of the bolt to make it ride correctly.This was on a Wiselite receiver, not a DSA; they may be different! That info has for some reason, gone away from his site! Good luck on your build!
 
#19 ·
hey VZ58, have you shaped and clearanced the ejector yet? the bolt will hit the ejector just before it goes into battery....if you have a factory rec piece with the ejector in it look at the differences between the two, otherwise metal needs to come off of the backside opposite of the face(case contact side)....height should be fine
if ya need a pic LMK
 
#20 ·
What is the Vector FCG like? I always heard you had to replace the buttstock for theirs.

The Height of the carrier matters a good bit. Make sure the bolt can strip a round from the belt, the gas piston can go all the way and the roller on the top of the carrier is riding in the top cover right.

Wish I could help, my parts might be different but if you need a measurement I can get them.
 
#31 ·
Pirate, here are the pics









Not included are the springs and pins.. The trigger is in the lower forgot to pull it out for photos. I know I am definitely missing at least one pin. There is no pin small enough for the striker release (little piece in the last photo)

Is this one of your sets? Cause I cant figure out the trigger coil spring...
 
#34 ·
Yes the receiver could be threaded, but it's the timing thing that most people would have trouble with or don't want to pay someone to do it, it's just easier to make it a press fit.
 
#35 ·
I would think with all the other firearms that require timing this would be the same. I can't help but wonder if someone somewhere in the reverse engineering of semi-auto RPD receivers preferred to be lazy and everyone else ran with it?
 
#36 ·
I was wondering the same thing about the threaded barrels.
I guess when it is said they have to be timed, this refers to the position of the barrel in the receiver when it is headspaced correctly. Then the cut for the extractor would have to be marked and then the barrel removed and the cut in the barrel made, then screwed back into the receiver.

Could anyone tell me if this is correct?
 
#37 ·
I'm guessing you have a new barrel?
If you do and there's nothing cut or drilled (gas port, ect) it's a lot easier to have a thread in barrel.
But yes your correct. Thread the receiver to match whats on the barrel, screw in the barrel and set head spacing, mark the barrel so you know what is top & bottom, then remeove it to cut the extractor groove and if it's easier to put on the other parts.
Now if the gas port is cut, you need to time/clock the barrel.
 
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