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RPD with a hammer fired adjustable trigge
Weaponeer Forums : RPD

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  cal50

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Posted: June 23 2018 at 9:28pm | IP Logged Quote cal50

My build project was sidetracked due to the unexpected death of my dad. Anyone that lost one of their parents knows the feeling and it’s just a funk to get thru. Getting my mom taken care of was / is #1 and tying to ease back into "normal" activities and work just feels weird.

 

 

That being said now back to my build~

 

The RPD is an awesome one-man portable belt fed with the only handicap is the semi auto versions with a funky striker like trigger mechanism.

The striker system yields a LONG trigger pull with a mushy trigger that is prone to not working more that it does work. The opposing springs (striker and recoil spring) are working against each other with a sweet spot for function. Vector arms did a different flavor of it with a custom made butt-stock and different spring set up that seemed reliable with most offered after it was a different version with mixed results.

 

With the nice parts sets available now and the limiting factor the striker trigger my shooting bud and I picks up a couple parts sets to build but we wanted a real trigger system. Something independent and isolated that did not affect or change the recoil spring rate. We had the same idea and looked at different options. My bud being a retired engineer he had the advantage of more free time to look at things while I had an idea to use AR parts. After looking at all the issues I spent a fair amount of time trying to make the AR fire control parts "work" which I could but every time there was a compromise and another problem. My friend picked up a Savage 775 trigger and it’s the base trigger for both of our builds.

It’s a perfect match for the RPD and the trigger action is unlike any other RPD out there. A three-way adjustable trigger with a CLEAN and repeatable break.

 

 

This is the foundation.

 

The top trigger group has already had the hammer and sear modified. The steel trigger housing has been milled down to fit inside the RPD grip frame. The stock hammer has been modified for profile and new sear catch machined with a few other minor tweaks.

 

 

Stock Savage 557 trigger~

 

 

 

I love this little guy~

 

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  cal50

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Posted: June 23 2018 at 9:29pm | IP Logged Quote cal50

Modding the RPD grip frame along with the Savage trigger go hand in hand. I basically milled out the pocket in the RPD frame deeper to accept the new trigger pack.

 

 

 

 

Getting it roughed in~

 

 

 

 

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  cal50

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Posted: June 23 2018 at 9:31pm | IP Logged Quote cal50

Once the Savage trigger housing slip fits inside the RPD grip frame you can position it correctly.

 

I modded the hammer and sear geometry so the hammer nose is .090 above the top of the sear so the slide will re-cock it. The stock hammer can be modified but you do not want it binding up so watch your clearances.

 

Long hammer looking at sear angle and travel limits~

 

 

 

Once the hammer is milled down for correct travel you can cut your sear notch in the hammer.

My friend used a trigger housing he made the same width to the front of the RPD grip frame where I used a steel Savage 775 trigger housing full length and some other mods. I wanted a stock AR hammer spring in my trigger where he used a smaller spring for his hammer. By using the full size trigger housing I had more room than his but I will need to make a cover for mine where his looks more streamlined. More about that later.

 

The RPD bolt carrier / slide is tapered and will hit on the sear nose if not modified.

I simply used a carbide endmill and put a slot for the hammer nose and sear clearance down the RPD slide.

Its a simple operation and allows you to slide the trigger pack up close to the slide. You need it for the hammer arc to hit the firing pin transfer bar that is common to all semi RPD conversions.

 

 

 

 

Since I am not using the factory RPD safety I decided to cut a couple filler slots and weld them in and fill in the other not use pin holes. Roughed in~

 

 

 

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  cal50

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Posted: June 23 2018 at 9:33pm | IP Logged Quote cal50

Next up is the important part of positioning the trigger pack in the RPD grip frame and welding it in place.

Its thin in spots but easy if you have a TIG welder.

 

After I welded my safety filler plates in and cut the pocket into the RPD frame ( thru the trigger guard) the grip frame is thin and wants to twist and warp. I did some straightening so it slid back on my DSA receiver.

 

The fit of the Savage trigger inside the RPD frame is firm but easily moved and clamped.

I slid it all the way up until the top of the sear made contact with the slide then dropped it down .040"

 

I have about .095" travel past the sear catch point on my sear travel and trigger reset and .040" off the slide gives me about .060" hammer contact and reset travel past the point the sear engages.

 

You can see the sear in the slot in the bolt carrier.

You do not want it to touch or be too low and not cock the hammer.

 

 

 

 

 

Took a break and tripper checked my position~

 

 

 

 

 

Clamped and final check and ready to weld.

You can see where I filled in the stock RPD safety window......

 

 

You can also see where I opened up the Savage hammer spring pocket~

 

 

 

 

 

I tacked welded it in several spots , removed the clamps then made a pass down both sides.

I also re-attached the trigger guard (welded) it to the Savage trigger housing.

 

Its all one piece now~

 

 

 

After welding you might have some heat warp. I used some NAPA lapping compound and it made the new grip frame and trigger fit perfectly smooth to the receiver.

 

 

 

This is as far as I have traveled thus far but the hard part is done.

 

The rest is basically the same used in all the other semi auto builds by putting a firing pin transfer pin in place, pressing the barrel and setting headspace.

 

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  cal50

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Posted: June 23 2018 at 9:34pm | IP Logged Quote cal50

Progress- grip-frame and trigger group complete.

RPD grip frame pass thru hole opened up for push rod clearance, push rod finished machined along with the recoil spring drive rod shortened and fitted.

Push rod started as a standard bolt~



Machined in thickness to fit the bolt carrier along with clearance for the hammer to pass thru and the hammer spring~



Stock recoil spring rod was shortened to clear the Savage 775 hammer and sear.
The length along  with the hole drilled in the rear of the push rod are identical spring pressure as the stock RPD bolt and spring. I scribed a line on the stock rod while is was seated on the bolt carrier even with the grip frame rear surface. Once I cut the stock rod down in length I drilled the correct hole depth in the rear of the push rod to match its seated length / position.

What it looks like modded~



Rod inserted in push rod~




This is the stock hole in the RPD grip frame than needs opened up to clear the push rod.



Roughing it out~





This is what you want when you are finished~




The frame still retains the stock rod and plunger~




Push rod in place~






Hammer test.
Hammer cocked~



Hammer dropped~

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  cal50

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Posted: June 23 2018 at 9:47pm | IP Logged Quote cal50

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  cal50

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Posted: June 23 2018 at 10:00pm | IP Logged Quote cal50

Sorry , having trouble embedding video~
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  backbencher

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Posted: June 23 2018 at 10:08pm | IP Logged Quote backbencher

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  cal50

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Posted: June 23 2018 at 10:13pm | IP Logged Quote cal50

What tags for embed ?
(thx)
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  backbencher

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Posted: June 23 2018 at 10:17pm | IP Logged Quote backbencher

Impressive.  Thanks for posting up.  Sorry to hear about your Da.
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Posted: June 23 2018 at 10:19pm | IP Logged Quote backbencher

[ youtube ] http:// www.youtube.com / watch?v= Wife Beach Video Take Out All The Spaces [ /youtube ]
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  cal50

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Posted: June 23 2018 at 10:25pm | IP Logged Quote cal50

backbencher wrote:
Impressive.  Thanks for posting up.  Sorry to hear about your Da.



Losing a parent sucks no matter how many times you think it thru.
Thank you~
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  Paraquat

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Posted: June 25 2018 at 9:34am | IP Logged Quote Paraquat

Very impressive work.
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  cal50

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Posted: July 15 2018 at 7:57pm | IP Logged Quote cal50

Making progress and did some live fire.

 

Completed the rest of the normal semi auto mods to the RPD parts. I did plug and TIG weld the hole in my slide so my transfer pin could be machined without deflection.

 

 

 

I installed a .187” diameter transfer pin to transfer the hammer drop to the RPD firing pin. I could have positioned my transfer bar a lot lower but it’s near the top of the hammer arc for max transfer of energy.

 

I used a 3/16” dowel pin to trim to length for a gauge to get my hammer drop at 90 degrees. Once I had this length I made a pin from O-1 tool steel. I put a “dimple” head on it for more surface area that the hammer strikes. I cold formed the head like a rivet then squeezed the sides in my milling machine vice to clear the cut out in the RPD grip frame at full recoil. A roll pin retains it in the bolt.

 

 

Notice the big head~

 

 

 

 

 

 

 After I headspaced the barrel, pinned it and got my slide done it was off to the range.

No surprise as the trigger functions, feels great and the RPD was running well. I ran some mixed ammo of Wolf ball and some hollow points. The hollow points made it choke instantly. This was an easy fix by taking the top cover and feed tray off and adding a slight amount of chamfer to the DSA receiver feed ramp. There was a step between the DSA feed ramp and the feed tray.

 

On my 1st test fire session the gun runs fine but I was not satisfied with some primer flow and cratering that I deemed heavy. My 1st test fire session I used a standard AR hammer spring with ½ a loop removed to fit in the Savage trigger housing.

 

 

 

I had an extra power AR hammer spring in my parts box and wanted to do the same test with the extra power spring to see if it made any change in primer flow. After running 25 rounds thru the gun (mixed with hollow points that ran 100% ) I collected the cases and compared. It appears the extra power spring did help some unless it’s my imagination. Less primer flow but still some cratering.

 

 

 

Rather than take a chance of blowing a primer I decided to bush down the bolt face and use a smaller diameter firing pin.

 

Stock bolt firing pin hole is huge....

 

 

Boring the bolt face out for a bushing ~

 

 

Much better now and less primer flow~

 

 

Stock firing pin turned down to .070" diameter

 

 

 

The only problems test firing is NOT having most of my pins staked in place because I want to tear it down and finish it so I have to watch the pins walking. I also must install the awesome Polish scope mount.........

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  bikergunnut

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Posted: July 15 2018 at 8:06pm | IP Logged Quote bikergunnut

Try superglue on one end of the pins for temporary hold. Melts with acetone.
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  cal50

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Posted: July 15 2018 at 9:29pm | IP Logged Quote cal50

bikergunnut wrote:
Try superglue on one end of the pins for temporary hold. Melts with acetone.


I have some wicking grade loctite but I am close to pulling it apart to finishe it so I will baby it thru.......
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  backbencher

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Posted: July 15 2018 at 11:01pm | IP Logged Quote backbencher

Heh.  I've had ARs for 13 years, only recently did I get around to installing the rear takedown pin detent.  Never lost a takedown pin yet.
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  cal50

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Posted: July 16 2018 at 8:56pm | IP Logged Quote cal50

One more "to do" task off my list, attached the excellent Polish optic mount.
This mount really is a work of art and not common. I had a nice RPD parts set I was going to do my build on but when the Polish RPD's with the original optic mount came  available I grabbed one and soooo happy I did.

You need to drill a couple dowel pin holes thru the receiver then three metric thread holes to attach the mount.

Set up in milling machine~




Original mount screws were included. The staked ends were cut off and need chamfered to use.




Attached to gun mount open ~




Mount closed.
I put the mount where it was comfortable to me . I bobbed the front of my MI mount.





Optics mounted~



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  ryche

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Posted: July 18 2018 at 9:22am | IP Logged Quote ryche

WOW just WOW...
Great work.
Mine is one of the striker fire combo between Egypt. and Chin. parts. I had the chin kit and bought the egypt. to get the receiver.
Still working on a small feed issue.
Sorry to hear about your dad, I lost mine a year and a half ago
sudden circumstances as well. It does take a while.
Great build, i need to go through the whole a little closer when i have more time to look at the pics.
Ryche
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  cal50

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Posted: July 18 2018 at 11:36am | IP Logged Quote cal50

It's worth the effort.....
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