Is there a mathematical way to calculate bullet drop from a 16" barreled semi auto rifle without a chronograph? Most of the ballistic charts use a 24" barrel for their testing.

I have a 16" PTR 91 with a Vortex Viper 1 X 4 scope. This obviously is not a long distance gun. More of a social unrest/plinking gun out to 300-400 yards. I still want to know where my bullet is going to hit at all of the distances out to 400.

Is 200 fps difference from a 24" to 16" barrel going to make a significant difference on bullet impact at these distances? I am looking at a 200 yard zero with the Hornady 155 AMAX cartridge.

Thanks in advance for your help.

Jbmballistics.com is your friend.

There may be velocity data for various loads and barrel lengths on the Internet. Perhaps you can find something close enough to give you an estimate of what you need.

But to give a rough answer to your question about how a 200 fps difference of the 155 grain A-MAX bullet would affect the trajectory at longer distances with a 200 yard zero, I used the bullet under standard sea level conditions at 2700 and 2900 feet per second with a sight height of 2.0 inches. (Very) approximate drops according to the Applied Ballistics solver:

2900 fps
300 yards: – 7"
400 yards: – 21"
500 yards: – 42"

2700 fps
300 yards: – 8.3"
400 yards: – 25"
500 yards: – 50"

Estimates for the effect on muzzle velocity by changing barrel length vary quite a bit, but one figure that seems to be more popular than others is 20 fps per inch. The high end is 50 fps per inch. The drop from 24 to 16 inches would therefore be from 160 to 400 fps. I suspect that 200 fps would be reasonably accurate.

200 FPS will make a significant but not dramatic difference at 400 yards with a 200 yard zero.


There is a truing feature built into the Strelok Pro app that lets you calculate your real muzzle velocity without a chronograph by interpolating based on actual drop vs. expected drop.

However, you need to shoot a fairly long distance to generate good data.


Frankly for your purposes I would search the web for chronograph data for the ammo you are using from the barrel length and gun that you have. If you can't get that specific just use barrel length alone. You will be on target at 400 unless you are trying to shoot a gnat in the ass.

there is a guy that does ballistics by the inch where he cuts down a barrel one inch at a time and shots various ammo through it. I don't know the link for it but you may find he did the caliber you are interested in if you do a search for "ballistics by the inch" I wound up using 94% of 24in velocity for my 19" 6.5 Creedmoor rifle based on this and it has worked fairly well.

Ken

A sporter magnetospeed is $180. Buy a chrono, or go in with a buddy or two on one. It is an important piece of shooting gear. You can guess, or you can know.

Even with the best bullet and velocity data, charts and computers, there's only one way to check to be sure it's correct.

And you can do that without the data, charts or the computers.

For the serious shooter or someone who might become a serious shooter in the future, a chronograph is a better purchase than another gun for the “stable” or “carry rotation.” Once we start working with a chronograph and a good ballistics solver, our knowledge about shooting grows by leaps and bounds.

The only thing I’ll caution is to read the reviews and make sure we understand the capabilities and features of the various types. For example, I believe the MagnetoSpeed cannot be used with handguns. It has its advantages, but also other limitations. More expensive units like the LabRadar also have their distinct advantages, but to get answers to the type of question posed in this thread, much less pricey units will do.

Thanks for all of the suggestions,guys. I shot the gun yesterday and did some work at 50 yards. .78" high at 50. I will walk it out to 100 and 200 next time out.

What is your sight height over bore measurement?

Measure this as best as possible -- from center of rifle bore to center of scope.

Fritz, it looks to be about 3.5"

quote:

Originally posted by fritz:
What is your sight height over bore measurement?

Measure this as best as possible -- from center of rifle bore to center of scope.

OK, using a boatload of assumptions here:
- muzzle velocity of 2,650 fps. The lower MV is in line with test done by rifleshooter.com
- air density altitude of 2,000 feet above sea level. This probably covers current air conditions for you on the coast.

guestimates per JBM
.8" high at 50 yards
3.8" high at 100
5.3" high at 150
3.6" high at 250
zero at 300 yards
5.6" low at 350 yards
13.3" low at 400 yards

Thanks for doing the legwork Fritz. This is getting more complicated by the minute. I am going to take the suggestion from Cas. I am already dialed in at .78 for 50 yards. I will shoot 100 and 200 and see where the numbers compare to yours.