Posted: 3/10/2019 2:46:29 AM EDT [Last Edit: 3/10/2019 2:48:41 AM EDT by 60-Driver] ETA: With GP switching importers and now the inevitable arrival of the A3, the following modification is probably moot. I finished it a few weeks ago thinking I would never see an A3 in the near future and began writing this before GP's announcement last week. That said, the following is how I modified my Stribog from A1 to A3 to include folding stock and Geissele trigger mods...



The biggest thing initially holding me back from buying a Stribog was the straight blowback action. After playing around with CMMG's Guard, I got spoiled with delayed blowback. Once I discovered the A3 version existed, I decided to wait only to have Eagle Imports say they "had no plans to import it anytime soon". Although I had no desire for another straight blowback, I bought one anyway. I tried it out and while a little snappy, was nothing worse than some 9mm AR's I've handled.



I started researching the A3, collecting info & pics from the internet. From what I gathered, the A1 and A3 were the same except for a few components which looked to be interchangeable. I put emails out to Eagle, GP Canada, GP Slovakia, etc, all asking on availability of A3 parts. And all said no. But I did get an explanation as to why; essentially had to do with licensing, import rules, etc. So much for an A3 conversion. But a modification on the other hand...



I studied the pics I had, drew up the delayed action parts in CAD, made sure my measurements were good, bought some spare parts from Eagle, then headed to my shop.



Step 1, "Roller": Easiest part to make. Best guess, the A3's roller is 5-6mm OD. I decided to use 0.250" since all my tooling/collets are standard. Ordered a 0.250" HSS drill blank and cut to size.





Step 2, Charging Block: Milled slot for the roller, shortened to size, profiled sides like A3 (reduces weight), and bored out existing 10mm hole to 0.4375" (did this to allow use of different pistol action springs; 1911, Glock, Sig, etc, with spring rates from 11# up to 24#)







Step 3, Trunion. Used a ball nose endmill to cut the recesses, then smoothed down the rear edges:





Step 4, Locking Piece: Hardest part to make. Several 1/8" endmills gave their life in the replication of this part. 11.1mm (0.4370") HSS drill blank with a 40* angle cut (used 40* since 80* locking piece is very common in the MP5). Took some left over charging block, milled it down. Positioned rod into block, drilled two 0.125" holes for retaining pins to hold them together. (I believe GP does this to allow use of the same locking rod with different weighted blocks for longer barrels, etc). Discoloration is grease:





Step 5, Buffer Plate: Milled out existing spot weld, removed guide rod, replaced with shorter 0.250" rod. (in hindsight I should've just shortened the exiting guide rod since it's not much smaller at 0.236" OD)





Step 6, Action Spring: I ordered several to try. Only one that really fit (i.e, long enough) was from a full size Sig P320. I got the green spring (40/45/357) but also ordered a red spring (9mm) to try. Original Stribog (top), P320 green (middle), P320 red (bottom)





Step 7, Charging Handle "Shuttle": No clue what this piece is actually called. Scratch made from Nylon 6/6. "Legs" are bowed slightly to prevent it free floating in the receiver when bolt is locked back.





All laid out with Cerakote on the bare metal:





For comparison, still shot from YouTube showing a real A3's internals (video credit to Vins Cat):





In case anyone's interested, here's how the roller delay works (best I understand anyway):

Bolt closed. Spring pushes forward against locking piece, forcing roller down and "locking" it into trunion's recesses:





Cartridge ignites. Reward motion forces the roller out of the trunion which pushes the locking piece reward against inertia and spring tension:





Once roller is clear of trunion, the bolt is free to cycle as normal (roller is now riding on inside edges of receiver):





Stock: Started with Scorpion EVO folding brace & adapter and modified it to fit onto factory Stribog backplate:









Looks good but Form 1 came back waaay sooner than expected. So bye bye brace, hello stock. Tried UMP. Too long:





Tried the B&T, just right:





Trigger:

Nothing wrong with factory trigger, just wanted something different, plus it provides three 922r parts. With a few shim washers and slightly opened-up trigger hole, a standard AR hammer/trigger fit perfectly. Had a Geissele SD3G laying around so why not:







So does it work?? I shot 200+ rounds and had zero issues with the gun itself. It 'feels' softer than what I remember but would love to shoot it side by side with an unmodified gun to see if there's any substantial difference. All in all, I'm happy with the end result. The biggest thing initially holding me back from buying a Stribog was the straight blowback action. After playing around with CMMG's Guard, I got spoiled with delayed blowback. Once I discovered the A3 version existed, I decided to wait only to have Eagle Imports say they "had no plans to import it anytime soon".Although I had no desire for another straight blowback, I bought one anyway. I tried it out and while a little snappy, was nothing worse than some 9mm AR's I've handled.I started researching the A3, collecting info & pics from the internet. From what I gathered, the A1 and A3 were the same except for a few components which looked to be interchangeable. I put emails out to Eagle, GP Canada, GP Slovakia, etc, all asking on availability of A3 parts. And all said no. But I did get an explanation as to why; essentially had to do with licensing, import rules, etc. So much for an A3 conversion. But aon the other hand...I studied the pics I had, drew up the delayed action parts in CAD, made sure my measurements were good, bought some spare parts from Eagle, then headed to my shop.Step 1, "Roller": Easiest part to make. Best guess, the A3's roller is 5-6mm OD. I decided to use 0.250" since all my tooling/collets are standard. Ordered a 0.250" HSS drill blank and cut to size.Step 2, Charging Block: Milled slot for the roller, shortened to size, profiled sides like A3 (reduces weight), and bored out existing 10mm hole to 0.4375" (did this to allow use of different pistol action springs; 1911, Glock, Sig, etc, with spring rates from 11# up to 24#)Step 3, Trunion. Used a ball nose endmill to cut the recesses, then smoothed down the rear edges:Step 4, Locking Piece: Hardest part to make. Several 1/8" endmills gave their life in the replication of this part. 11.1mm (0.4370") HSS drill blank with a 40* angle cut (used 40* since 80* locking piece is very common in the MP5). Took some left over charging block, milled it down. Positioned rod into block, drilled two 0.125" holes for retaining pins to hold them together. (I believe GP does this to allow use of the same locking rod with different weighted blocks for longer barrels, etc). Discoloration is grease:Step 5, Buffer Plate: Milled out existing spot weld, removed guide rod, replaced with shorter 0.250" rod. (in hindsight I should've just shortened the exiting guide rod since it's not much smaller at 0.236" OD)Step 6, Action Spring: I ordered several to try. Only one that really fit (i.e, long enough) was from a full size Sig P320. I got the green spring (40/45/357) but also ordered a red spring (9mm) to try. Original Stribog (top), P320 green (middle), P320 red (bottom)Step 7, Charging Handle "Shuttle": No clue what this piece is actually called. Scratch made from Nylon 6/6. "Legs" are bowed slightly to prevent it free floating in the receiver when bolt is locked back.All laid out with Cerakote on the bare metal:For comparison, still shot from YouTube showing a real A3's internals (video credit to Vins Cat):In case anyone's interested, here's how the roller delay works (best I understand anyway):Bolt closed. Spring pushes forward against locking piece, forcing roller down and "locking" it into trunion's recesses:Cartridge ignites. Reward motion forces the roller out of the trunion which pushes the locking piece reward against inertia and spring tension:Once roller is clear of trunion, the bolt is free to cycle as normal (roller is now riding on inside edges of receiver):Stock: Started with Scorpion EVO folding brace & adapter and modified it to fit onto factory Stribog backplate:Looks good but Form 1 came back waaay sooner than expected. So bye bye brace, hello stock. Tried UMP. Too long:Tried the B&T, just right:Trigger:Nothing wrong with factory trigger, just wanted something different, plus it provides three 922r parts. With a few shim washers and slightly opened-up trigger hole, a standard AR hammer/trigger fit perfectly. Had a Geissele SD3G laying around so why not:So does it work?? I shot 200+ rounds and had zero issues with the gun itself. It 'feels' softer than what I remember but would love to shoot it side by side with an unmodified gun to see if there's any substantial difference. All in all, I'm happy with the end result.