The 1.275 is the MAXIMUM OAL for the cartridge, the MINIMUM OAL is 1.150 anything between the two is acceptable for function, but maybe not accuracy. Accuracy of a particular load will vary from weapon to weapon and load components to load components. The secret, if there is one, to reloading accuracy is that whatever you are doing, it is constsistantly repeatable, and you don't change anything, except intentionaly. Minor changes in seating depth can also change performance of your load, because it will change the dimentions of the combustion chamber, especially critical with propellants like Bullseye where target loads have low charge volume densities. If you don't keep everything the same, the point of impact will change from shot to shot and accuracy will suffer. The most important thing is that case length is constant, rimless semi-auto cartridges (exception is .38 Super Auto which is n't rimless it's semi-rimmed and headspaces on the rim like a rimmed revolver cartridge) headspace on the end of the case mouth, and the 45ACP maximum case length is 0.898 trim to length is 0.893. thats only 5/1,000 of an inch margin! Since the cartridge headspaces on the mouth of the case, you can't use a "rolled" crimp, it must be a taper crimp or "factory" crimp.
Pure wheel weights sound a little hard to me. I currently use 16 lbs of wheel weight metal and 1 lb of 50/50 solder this alloy will give you projectiles of about 87% lead 8% antimony and 4% tin the missing 1% is a mixture of low melting point metals and other things like calcium and arsenic, end product is about 19 or so on the Brinell hardness scale, and easy to resize, with no barrel leading at target velocities. I use a 35 year old RCBS Lubrisizer and Red Rooster lube. The Red Rooster has a high melting point so you have to use a lube heater under the Lubrisizer, but the finished product isn't sticky like the old alox/beeswax lubricated bullets, and you get very little lube build up in the seating die, which if not cleaned on a regular basis will cause bullets to seat deeper.
The lead gives you maleability, the antimony hardness, and the tin low viscosity so the mould cavities fill evenly without voids. Really more than 2% tin in bullet casting is probably wasted, but the above mentioned alloy is easy to make, except that 50/50 (lead/tin) solder is getting hard to find and more expensive when you do find it.
Years ago, in one of my other lives, I was a Bell System Cable Splicer, and Cable Repairman, and had almost unlimited access to lead calcium sleeves and other scrap lead alloys like wiping solder and seam, and sealing solder, my alloy of choice fit the available materials.
Keep tinkering, and don't forget to keep good notes about various batch results, a little tweaking here and there can have a great effect on overall performance. I found that 100 round test batches worked well.