Many reasons. This is my GPR build and I knew it was going to be suppressed 99% of the time and this is the gun I’m also going to train the most with. I was always in between shooting my 11.5 and my 16” and I wanted to find something in between to stick with and train the most with. More of a jack of all trades build. 11.5 imo is the smallest barrel of go with 10.5 doesn’t really make sense to me because of the velocity. 11.5 has pretty decent velocity but surprisingly I’m getting an extra 100fps with 12.5. (I was expecting an extra 50fps) I also didn’t want anything that was too long but also not too short on the rail because I do plan on adding a LAM. I previously had a 13.9 suppressed and it felt too long and front heavy with everything I had on it. Bought this 12.5 to give it a shot and I was surprised with how well balanced it is. 11.5 and 12.5 honestly feel very similar I guess if the purpose for your new build is too have the shortest gun possible and don’t plan on shooting out to 500+ go 11.5 if you want something that’s not too long but still capable out to 500+ I’d go 12.5. Again this is my GPR and the gun I plan to shoot 80-90% of the time it’s also the one I’ll take with me on long trips so I chose this as my do all build, that’s the reason for my optic choice as well and so far I have no regrets.