To get to 1250 fps and stay within a prudent margin of safety pressure wise you're going to need to use a somewhat slower powder with a burn rate in the range of Green Dot, Universal Clays, Herco, or as suggested, IMR PB, etc. Hodgdon's 'regular' Clays & E3 are both too fast to get there without really pushing the safe pressure limits IMHO, to put it in perspective, using a Winchester AA or Remington STS/Gun Club hull as a baseline, the manufacturers data pages for these two powders don't list a single 1 1/8 oz. load over 1200 fps.
All the powders mentioned above will do a fairly good job for your requirements. That being said, there is only one powder that I would reccommend to do the job: Unique. 22.5 to 23 grains of Unique will easily give you the 1250fps, and it is one of the cleanest powders I have ever shot. The best thing Unique has going for it is it is a very clean, fairly low pressure powder that consistantly throws excellent patterns. I currently have some REX II, but, once gone, you can bet I'll be shooting Unique. With the same basic reloading data as Green Dot, +/- 1 grain, it is a much cleaner powder. The cost is the same as Green Dot as well. Green Dot is a great powder, however, it is dirty and dirt is not semi-auto friendly powder.... Just my opinion.. Dan Thome (Trap2)
Unique has been around a long, long time like PB and has a world of load data established for it. I like PB for my 1100/s and 11/87/s and Unique for fast 1 1/8 games loads in my 870/s. It is very clean and low pressure if the right loads are selected. Unique also costs less than PB.
Either Unique or PB will give the results you desire.
My experiences with Green Dot have been very positive once you get the pressures up high enough to propell a 1 1/8 Oz payload to 1,200 - 1,250 FpS. I think this is really where GD works best and in this range it isn't a 'dirty burning' powder as many have observed with lower pressure loadings.
I have shot quite a few Green Dot loads though a 391 Beretta and didn't have any unusual fouling of cycling issues. Good performance and priced right with a long history of winning - what more could a guy want?
Hodgdons TiteGroup pistol powder. It takes so little that you need to use a 1 oz wad for a 1-1/8th oz load, but it works great in the full power loads. I is rumored that that was the powder in Nitro 27 shells. I do not know if that is true, but I like it.
The perfect 1 1/8 oz. load is hard to come by but we all keep trying.
Green Dot @ 1250fps. and up with a moderate primer is a good performer and Unique which you can also take up to 1300fps. Both are clean burning as previously stated at those velocities.
You can move your velocities up with Fed. hulls and keep pressure less than you can with AA or Rem. Clubs.
Just received load testing back from Precision Reloading and the Fed. hulls (not FGM), the SD and ES were all over the chart. Surprisingly, Herco @ 1300fps.in a Rem. Gun Club Hull was a top performer with a Rio primer. However, the other loads with Rio were also eratic. The top 1 oz. load was with E3 @ 1300fps.
Based on the above, I will go with E3 for the 1 oz. loads in a Rem. Gun Club Hull with a Win. primer and Unique for 1 1/8 oz. loads with a Win. primer, also in a Gun Club Hull. The increase cost of a Win. primer over a Rio is $.05/box but your performance is good and steady. FWIW.
Thanks for all the info. Will have to try some of the powders listed and see. Want the least amount of the backfire at the ejection port when shooting. Won't shoot this shell all the time only on the long stuff.