BC vs Terminal ballistics

[gunnyart]

Just got my first can and am developing a load. I've shot some milsurp FMJ and it isn't hearing safe.

Here is my thought. Either...

sierra pro hunter 170 flat point (b.c. 205 I think)designed to perform at lower velocities but probably not as slow as subsonic.

Or

168 barnes ttsx (.4xx b.c will not expand below 2000 fps)

My quandary is because my desired muzzle velocity with either bullet will be roughly the same (barely subsonic) which bullet makes more sense? slow and slowing faster but a bit more traumatic or maintaining a bit more speed and penetrating deeper and perhaps extending my range a bit.

I'm open to other bullet suggestions just thought I'd start with a couple I had on hand.

platform: Rem 700 PSS LTR 20" (1:12 twist if I'm not mistaken.)

 

[Greg Langelius *]

Re: BC vs Terminal ballistics

The way to treat subsonics is to imagine them as a pistol round. What makes sense as a subsonic pistol round bullet makes sense as subsonic rifle round bullet. Ask yourself, what bullet shape would you use for a .45ACP?

As Sierra hunting rifle bullets go, the Pro-Hunters are good for slower loads; but as you say, subsonics may not be fast enough.

Personally, I would prefer a 1:10" (or faster) twist for a .30 cal subsonic. The combination of lower velocity and slower twist rate could significantly limit maximum bullet weight(length) for successful stability.

 

[gunnyart]

Re: BC vs Terminal ballistics

Okay, so what would be a good "pistol" bullet weight and shape that would stabilize in a 1:12 twist 20" barrel?

 

[Greg Langelius *]

Re: BC vs Terminal ballistics

Well; the P/H 170 you mentioned would likely be good; and you might want to try the 180 and 220 to see how heavy you can go and still get reliable stability.

I have doubts about the value of boattail spitzers at subsonic velocities, and would prefer to put my money on a round nose, flat base bullet.

Stability can be an individual thing from barrel to barrel, and I don't want to give the impression I have all the answers. Things like bore finish and smaller bore diameter variations can make a difference. I'd suggest suspending one's surprise reflex on this one and seeing what actually works.

FWIW, I have even seen references to open base (lockbase, etc.) bullet being loaded base first and being used as a sorta thashcan HP. The resulting sorta-teardrop profile is actually pretty efficient in a subsonic velocity regime.

Greg

 

[KYpatriot]

Re: BC vs Terminal ballistics

I think a boattail as far as drag goes is actually more effective at subsonic velocities, but it will be difficult finding a rifle bullet soft enough to expand reliably at sub velocities.

I think there is a market here ready to be exploited by the bullet manufactures...if they were clearly marked as subsonic only bullets, a nice soft but aerodynamic heavy for caliber rifle bullet design would sell well. I suspect that the size of the market hasnt justified the development and production cost for such a round.

 

[MALLARD]

Re: BC vs Terminal ballistics

i'd check out some 200 grain bullets like greg said

id even look at some 100 grain bullets , the .30 carbine type , and load those behind 15-20 grains of pistol powder , those loads are usually pretty quite, ideal for 200y or less.

 

[gunnyart]

Re: BC vs Terminal ballistics

I guess without lots of ballistic geletin testing there is no way to know. I'm looking for a reliable starting point that at the very least will stay stable enough to avoid a baffle strike on my new toy.

I picked up some trailboss so I guess I'm out of excuses. I just need to load some up and see what happens.

One other question I've seen some debate about...

With a 70% case capacity starting load should I run magnum primers?

 

[Huckleberry75]

Re: BC vs Terminal ballistics

10.5 gn of Trail boss and the 170 FN shoots well in my 1/10AAc, but my go-to slow thumper is the Sierra 220gn RN and 12.2. Gn of Trail Boss. Fly's like an anvil and hits like the same. It is about as noisy as a .22CB cap. The 220 runs a consistent 1036fps at 80*F and 1500asl. Only problem is that it's 17moa lower at 100yds than my regular full house 168 load, even though it shoots about 1/2 moa. This is with LC brass and about any LRP. I did NOT enlarge the flash holes, and have not seen a need for a magnum primer.

Your subsonic projectiles need to have a weight forward/centered type design (ie Round Nose), and typically the boat tails are not optimal. There are some special SS boat tails built by Lapua (IIRC), but they are much more costly than the 170/180FN and 220RN options.

 

[bricktop]

Re: BC vs Terminal ballistics

These are really nifty, albeit expensive:

http://forum.accurateshooter.com/index.php?topic=3782985.msg36048220#msg36048220

http://lehighdefense.mybigcommerce.com/308-maximum-expansion-200gr-subsonic-bullet/

http://lehighdefense.mybigcommerce.com/308-controlled-fracturing-170gr-subsonic-bullet/

Haven't looked at stability in a 12-twist though.

 

[gunnyart]

Re: BC vs Terminal ballistics

Thanks, lots of good info. Short of a high speed camera how can you be sure your bullet is being stabilized? look for round holes in the target?

 

[gunnyart]

Re: BC vs Terminal ballistics

Digging through my reloading supplies I found a full box of Hornady 110gr sp's
Shouldn't have any troube stabilizing them.

I also found some Hornady 220 round nose. I'm sure they will hit hard but really nervous about baffle strikes from a 1:12 barrel.

 

[bricktop]

Re: BC vs Terminal ballistics

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: gunnyart</div><div class="ubbcode-body"> Thanks, lots of good info. Short of a high speed camera how can you be sure your bullet is being stabilized? look for round holes in the target? </div></div>

This helps to find if you are stable, marginally stable, or unstable:

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

For 12-twist at subsonic (~1040 fps), the first 200-gr bullet is definitely not stable. You'll need about an 8-twist to effectively use it. The second 170 grain one is marginally stable for 12-twist (10-twist would be better), which means it may be stable for some atmospheric conditions, or may not be able to stabilize it.