I just loaded , a box of my favorite .38 special J frame loads.
I used 4.5gr. of Unique, with a 158gr. cast SWC. my little 3" model 36-1, loves this load. it is not plus p, so its easy on the gun & very accurate & little to no leading. In my model 10-8, I bump the powder up to 5.5gr. & it is plus p, & very accurate.

I'm not a fan of Unique but I'm sure it's a good load like you said. I make my .38 Special +P loads with HS-6 and standard pressure loads with W231.

What kind of velocity does that 4.5gr load generate from a 3" barrel?

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SWCA #3437

I havn't had a chance to use my chrony on the J frame load on them yet, but my model 10 load averages 1035 fps.

5.4 grains of Unique with that bullet in my 2" J frames runs around 890 fps. Oddly enough it does the same in the Model 15 (4") but runs right at 1000 in the 4" M&P, which is what you would normally expect.

For whatever reason(s), this M15 is a "slow" gun. I have a 4" 686 that is the same way.

B/C gap will effect velocity. also using cast bullets vs jacketed, or how hard the cast bullets are.

I have a M625JM, 4" 45ACP that consistently delivers the same velocity as a 5" 1911 with the exact same loads.

I'm not so sure that the barrel gap is a big deal to be honest. It sure doesn't prove out in those guns.

As for 38spl and 3" J frames, I have some loads that deliver 1100fps from that length tube, and yes, they are in 38spl cases!

I've often wondered what part wear on tooling used in the rifling process plays. Ditto for cylinder chamber reamers.

My FBI Replica load does an AV of 957 fps from a 4" M686 and an AV of 889 fps from my 2" M640.

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SWCA #3437

I use the 4.5 grain Unique load with the 158gr LSWC and a CCI primer all my 38s love it. That includes 3 Js, an Official Police and a couple of Smith 357s. And a SP 357.

That load shoots well and to point of aim in my fixed sight guns.