Subsonic and Accuracy

[KevinThomas]

If you look at Sierra's BC ratings, they are nice enough to give you a BC for specific speeds. Other than some handgun bullets, most of them have a higher BC when pushed faster.

That’s not entirely true. Sierra lists multiple BCs as a way of force-fitting thei bullets into a drag model (the G1) which isn’t suited to most of their bullets. Most of their line would be much better served bye the G7 model, which Berger and Lapua use for most of their match type bullets. The Sierra multiple BC bands were developed by Bill McDonald and Ted Almgren as a means of better matching the actual resulting trajectories to a drag model that they knew wasn’t a good fit. At that time, virtually everyone in the industry simply used ththe G1 as a universal default drag model, accepting the inherent inaccuracies of the resulting trajectory predictions. It was a good attempt to rectify a problem they were well aware of, without resorting to using the more appropriate G7 model. At that time (30 years or so ago) very few shooters had any understanding of BC as a general concept, much less the various drag models appropriate for a multitude of different bullet profiles.

Hope that helps a bit.

 

[Outrider27]

My 45 ACP has sub sonic, MV I would have to hold over about 35 ft at 500 yards.
God only knows how much wind deflection

Big bore handguns do tend to have huge mid-range trajectories, lol. Never shot mine beyond 200 meters (IHMSA big bore matches), and even the .44 Mag had a mid-range trajectory on the order of 4 feet at that range with a max load of 2400 pushing a 240gr bullet - the .45 ACP made that look puny. Much preferred my 7BR XP-100 or the 30-30 Contender, which had laser trajectories compared to that (high single digit inches).

But Cpl. Harrison was dealing with the same things; my calcs showed he hit max elevation at about 1600 yards into the 2703 yard flight, and that the distance above line of sight was a bit over 150 feet (!). That figure really got my attention....

 

[rebel]

Question...and maybe the answer to my question may help explain the transition from super to sub.

Why is it so hard to shoot small long range groups with subsonic loads? A while back a guy posted some video shooting a 500 yard target with a 300 wisper. Looked to me like 6 -10 FEET of vertical at 500 yards. I thought is was poor, but the response was..."try it with a subsonic load some time". The wind gets you. My response was that would give you horizontal, not vertical. I don't remember where it ended up.

Kind of put damper on my dream of a suppressed 338BR shooting 300's.

Thanks,
Tod

Because of the powders or reduced loads you need to shoot subsonic, you get ES's that can be very high. That results in your vertical. Once you have that as tight as you can get it, think of the trajectory - a 220 MK from a 300 Blk is around 18 inches high at 100 yards if zeroed at 200 yards. At 150 it's dropped a lot but not as much as it's going to in the next 50 yards. Instead of a rainbow, you have sort of quizzical eyebrow trajectory. Most rifles shooting these cartridges are not bench guns, therefore do not slide in the bags and return to zero. You have to aim in the same spot, in the same way every time. In other words, you have to work for it because the rifle isn't going to do it for you. Shooting at 200 yards with a heavy sub is akin to shooting at 1000 yards with a conventional super - except the sub never enters transonic, just starts falling out of the sky. Controlling that fall is tough.
I don't even want to talk about wind