'25 is a bit slow for a case of that capacity. That's why you're getting a decent muzzle flash and low velocity. R22 would produce better velocity, regardless of your barrel length. The muzzle flash is just hot gases igniting again when they hit the fresh oxygen in the air, rather than powder kernels that are still burning when they leave the muzzle (which is a commonly-believed myth). Powder burn rate should be matched to the powder capacity-to-caliber ratio, as well as bullet weight.
Here's how you can know for sure (I've seen a few examples of this experiment in the past, and the results are always the same): Get a 26" barrel installed on that RSAUM and chrony the velocity with a max, but safe, charge of R25 and R22. Then chop off 1" at a time and re-chrony the loads. You provide the barrel and I'll provide the popcorn