Effect of BC on subsonic bullets

[LouBoyd]

This chart might be of interest. Here are the ranges which a bullet fired at 1000 fps will drop to 800 feet per second as a function of it's Ballistic Coefficient (BC). It assumes the target is at the same elevation as the rifle and the atmosphere is standard metro. These are G1 BCs in the 800 tps to 1000 fps range (as calculated by McDrag). Dropping from 1000 to 800 fps retains 64% of the muzzle energy for any bullet. The chart is not dependent on bullet weight or caliber (though BCs are).

BC yards for velocity to drop from 1000 fps to 800 fps
1.0 1520
0.9 1360
0.8 1180
0.7 1080
0.6 860
0.5 720
0.4 580
0.3 420
0.2 280
0.1 140

To me this indicates the minimum BC requried for various ranges of subsonic shooting. You can choose a smaller or larger value for acceptable energy loss. The maximum practical range can also be limited by wind deflection and vertical stringing due to muzzle velocity variations. All three need to be conisdered.

[MIKEONE]

Wow thats pretty interesting.

So a G1 bc of 1.0 will reach 1520 yds at 800fps, when fired at 1000.

The drop would be -4475.2in lol but the fact it retained that much velocity is amazing!

Thanks Mike

[Spook]

Almost 90 days later and rereading it a hundred times.... this finally sunk in. I need to attend "Remedial Reloading 101".

[LouBoyd]

Perhaps my chart was somewhat (or a lot) misleading. i was not trying to say that by using high BC bullets it becomes practical to shoot subsonic at ranges of 1500 yards. In most cases the practical range for a subsonic bullet is limited by vertical stringing. This is because vertical drop is exactly proportional to the >square< of the time of flight. I only posted the chart to show the effect of BC on delivered energy.

Here's a more realistic comparison. Assume two 458 caliber bullets, one with G1 BC in the 1000 to 800 fps range of 0.4 (cast lead Postell?) and the other with a BC of 0.8 (copper alloy VLD). Assume they're both 500 grain. Assume you can load them both for for a 1% extreme velocity spread (995-1005 fps) and assume you can estimate average cross wind velocity within 2 mph. What group size should you expect vs range?

Parameter 0.00 200 400 600 800 1000 range to target in yards (for all below)

Velocity drop resulting from atmospheric drag
.4 velocity 1000 915 849 793 744 600 feet/sec
.8 velocity 1000 986 976 915 849 820 feet/sec

Energy (1/2 MV^2)
.4 energy-- 1100 930 800 698 614 542 ft-lb
.8 energy-- 1100 1011930 861 800 747 ft-lb

Angular vertical drop relative to bore sight
.4 vertical 0.00 17.3 56.3 99.8 148 200 MOA
.8 vertical 0.00 16.5 52.6 90.7 131 174 MOA

Horizontal group sizes in inches from wind deflection
.4 wind d 0.00 1.0 3.9 8.5 14.9 23.1 inches for 2 mph estimation error
.8 wind d 0.00 0.5 2.0 4.4 7.7 11.9 inches for 2 mph estimation error

Vertical group sizes in inches from velocity dispersion
.4 stringing 0.00 1.4 5.5 12.0 422.4. 34.5 inches for 10 fps mv variation
.8 stringing 0.00 1.5 5.4 12.1 22.2 33.8 inches for 10 fps mv variation.


Conclusions:
Doubling the bullet's BC from .4 to .8 has the following effects:

1. It results in a moderate increase in delivered energy.

2. It educes overall drop but only by a small amount. Drop at ranges in excess of 600 yards requires specialized scopes with very large vertical adjustment range for either bullet. (A Leupold Mk 4 16x40 will barely make 800 yards with a .8 BC bullet).

3. Wind deflection is reduced close to inversely with BC. This is the most important reason to use high BC bullets.

4. BC has negligible effect on vertical stringing at least between .4 and .8 BC. Note that the effect favors high or low BC bullets at different distances. This is the result of the shape of the G1 drag model which is not a smooth function as normally implemented in ballistics programs. It isn't easy to handload subsonic ammo 1% extreme spread (10 fps) but it can be done.

How far you can shoot with subsonics becomes a matter of:
1. how large of group size is acceptable for your application?
2. how well can you measure or estimate the effect of crosswinds?
3. how small of velocity dispersion can you achieve with your loads?
4. How much energy do you need at the target?

I'm showing no error for distance measurement or atmospheric density since both can be directly measured accurately. Nor for aiming or rifle quality. Those should be negligible

[Expatriot]

That seems like an awful lot of math to tell us what we already know that the maximum practical range of a subsonic bullet is going to be about 300 yds.

[Spook]

Originally Posted by Expatriot

That seems like an awful lot of math to tell us what we already know that the maximum practical range of a subsonic bullet is going to be about 300 yds.

Funny, I shoot my 510 to 600 with regularity,pretty accurately too. Working on getting there as accurately with my 338 .

[LouBoyd]

Originally Posted by Expatriot

That seems like an awful lot of math to tell us what we already know that the maximum practical range of a subsonic bullet is going to be about 300 yds.

Practical range of any firearm is determined by the size of the target and the amount of energy needed when the bullet arrives. Many subsonic rifles can easily reach 1000 yards with lethal energy. The primary limitations on accuracy are vertical stringing from velocity variations and wind deflection.

[Expatriot]

Many subsonic rifles can easily reach 1000 yards with lethal energy.

There is a big difference between reaching a distance and being able to hit the target you are shooting at.

The effective or practical range of a bullet or cartridge combination is the distance at which you can hit the thing you are aiming at. The energy the bullet is carrying is pointless if it doesn't hit the target.

[Spook]

Originally Posted by Expatriot

There is a big difference between reaching a distance and being able to hit the target you are shooting at.

Only if you miss.
Your statement is analogous to saying there is a big difference between how fast a car will go and how long it will go that fast?

Originally Posted by Expatriot

The effective or practical range of a bullet or cartridge combination is the distance at which you can hit the thing you are aiming at.

Which you'll have to agree is further away for some than others.

Originally Posted by Expatriot

The energy the bullet is carrying is pointless if it doesn't hit the target.

Conversely it makes all the difference on earth if a target is engaged successfully.


You're not trying to argue the credibility of what Lou has posted are you?

[LouBoyd]

Originally Posted by Expatriot

There is a big difference between reaching a distance and being able to hit the target you are shooting at.

The effective or practical range of a bullet or cartridge combination is the distance at which you can hit the thing you are aiming at. The energy the bullet is carrying is pointless if it doesn't hit the target.

Sure, but what is your target? A prairie dog? The vital zone of a deer? An enemy soldier? An enemy troop transport plane on a runway? A 100,000 gallon fuel storage tank?

You say "thats a lot of math" but it's really not much if it frees me from the mindset that subsonics are only good for 300 yards.

I trained on the M67 recoilless rifle in the US Army (late 1960's which only has a 700 fps muzzle velocity. It had a nominal 400 yards effective range against tanks but considerably further against bunkers. It was useful as an anti personnel weapon to well beyond 1000 yards and more accurate than a mortar.