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Posted: 3/9/2026 1:26:03 PM EDT
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I’m trying to decide if it’s worth the effort to install a FRT in my 1980’s era Colt DOE 9mm, blow black operating system. I’m not sure if it makes a difference but I am using the hydraulic buffer in my 9mm DOE. Thoughts or Recommendations? |
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Had my AZR'd, Blowback9 GRS'd DOE at the range yesterday. I am still smiling. It was my first GRS install. Dropped it in the DOE 'cause I was taking it with me to the range. It's gonna live in there now that I realize the long buffer tube doesn't work with the braces on my other 9s. It is lots of fun. I use the 3 pos AZR in it 'cause easy button. Did you go full GRS? I did, so I swapped on a Just Right Carbine buffer tube (+1" to accomodate a pair of stainless weights. Thought to add, mine is the H&R clone. |
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Originally Posted By getchasum: I'm trying to decide if it's worth the effort to install a FRT in my 1980's era Colt DOE 9mm, blow black operating system. I'm not sure if it makes a difference but I am using the hydraulic buffer in my 9mm DOE. Thoughts or Recommendations? Assuming the lower isn't blocked and has typical .154 FCG pins it could be as simple as dropping in a cassette FRT like the Partisan disruptor or something like the Arizona regulator. These generally have good timing for blowback. I would stear clear of any lever type FRT like Atrius, ARC-FIRE, MARC and super safety. These can work and usually do work OK, they have the widest timing range and this can cause lots of problems in blowback guns. Your hydraulic buffer should do a reasonable job at stopping bolt bounce. Is it an enidine or kynshot? If enidine carbine length, you likely have a spacer at the end of the buffer spring? If yes, consider adding a kynshot, 2.5oz spacer weight replacing the spacer. The older colt bolts were on the heavy side, usually 16oz or a tad more. This is a good thing but the enidine was like 4-4.5oz IIRC so your on the light end at 20-21oz of what's now considered enough total mass for AR 9mm's, 22oz and up is considered safer. My FRT AR9's run around 25-27oz of total mass. I do use a few kynshot 9mm hydraulic buffers as well as tungensten weighted 9mm extended buffers. Assuming the lower is GTG or you're willing to have what ever colt did undone... adding an FRT is doable and lots of fun 
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The cassette type FRTs (Partisan Disruptor and Rare Breed FRT-15L3) and the Arizona Regulator are particularly well suited to blowback ARs, because of how well they control the timing of the hammer drop. For good function, you're on the right track with the hydraulic buffer. I use a Kynshot 5015HD in my Glock mag FRT 9mm. But you want to get that bolt/buffer combo weight UP. The heavier, the better. The Kynshot buffer weighs only 6 ounces, compared to the excellent KVP 10 ounce buffer with 4 tungsten weights. I use the KAK tungsten pinned bolt weight, with a KVP bolt, and a Kynshot 5015HD buffer, for a combined weight of 22.7 ounces. You need to ensure that your bolt has a full auto profile tail like this:  " />If using a Rare Breed or Partisan, you do NOT need to modify the pinned bolt weight. If you're using an Arizona Regulator, or Hoffman Super Safety, or Atrius, Cornfire, ARC-Fire, or another lever style FRT, you will need to cut your bolt weight like this:  " />The ejector is the Achilles Heel of 9mm AR builds. It needs to be just right, and Shulz (posted above) will help you get it nailed down. |
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I wouldn't do it to an original Colt now that the H&R clones are available, but I am tagging this thread out of interest in doing this to my DOE and 635 clones. I use VLTOR A5SH3 buffers (8.81oz) with the spacer removed rather than hydraulic buffers. The H&R 9mm bolts are 14.3 oz, which gives me a combined weight of 23.11oz. The A5SH4 (9.6 oz) buffer would also work, with a combined mass of 23.9 oz; as would the A5SH2 (7.97oz) buffer with a combined weight of 22.27oz since they also get you to the 22-24oz sweet spot for blowback 9mm reciprocating mass. That's not necessarily true for Colt bolts, because they use a slightly different design with an internal extractor. |
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| Thanks for the pic! Was just here about this. I built mine about 15 years ago running a weighted bolt with A5H4 buffet and rifle spring in a standard car length tube on colt sticks. After playing with a few Atrius’s, I decided it was time to try a 9mm. Mine has always shot everything, ball, HPs, steel case perfectly, so was afraid to mess with it without some research. I got an AZR 3pos, but it wasn’t working right. Figured it was trimming the bolt weight needed, but this really helps! |
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