Thread: Trajectory.

Because I'm restricted in available range to sight in my rifles can anyone tell me what the trajectory for my 7.62x39,123gnsp doing 2380fps would be if sighted in @ 50yds. Also a 150 gnsp 308 doing 2750fps. In both cases where would the point of impact be further out? I only need it on a rare occasion when I couldn't stalk closer for a shot. For example; If i were to sight a 30 06 in with the same 150gn projectile @30yds it would be close to on @ about 250 - 300yds.
Phil.

Sorry guys. forgot to mention they're both Howas, one a mini action the other mod1500 & barrel length's 20" with GW suppressors

JBM - Calculations - Trajectory

What projectiles? need to know for the bc

Having sighted in both my 7.62x39 and 308 at 50 meters the 7.62x39 will be bang on at around 160m ish and the 308 around 200m ish.

Back in the AR15 days, many on us used to sight them at 50m and this would have them about bang on at around 200m ish.

A ballistic calculator will give you exact numbers.




Sent from my iPhone using Tapatalk

I have done exactly the same thing for mine. The bc of the projectile I used was .282 and my measured speed was 15 fps greater than yours.
These are the figures assuming a scope height of 1.5 inches above bore
50 yds zero
100 yds zero
150 minus 2.5 inches
200 minus 7
250 minus 14
300 minus 24
I have verified this out to 300 and those are the adjusted figures. If you have a half mil dot scope I have a Nikko sterling one your drops are
150 half mil
200 one
250 one and a half
300 two.

For sighting at such a short range there are 3 critical pieces of information - BC, speed, and the exact height of your scope above the bore. JBM is probably the easiest available trajectory calculator.

I sight my 7.62x39 inch and a half high at 50mts and its about bang on again at 200mts.

Greetings Tararua Phil,
There are a couple of factors that make longer range verification of your zero advisable. The first is drift, called spin drift in the US. Drift is due to the rotation of the projectile as it travels through the air. Air pressure is slightly greater under the projectile nose than above and this causes the projectile to drift to the left or right depending on the direction of barrel twist. Most military rifles are zeroed at 300 metres or yards partly to deal with drift. The second is the multiplication of any error at shorter ranges at longer range. Most non parallax adjustable scopes are adjusted to be parallax free at about 150 metres. These scopes will show some parallax at 50 yards or metres. Parallax is the apparent movement of the cross hairs on the target due to head position.
With your 7.62 x 39 and the ranges you shoot at you may not notice any difference but it could be a problem with the .308 especially if you stretch the range a bit. Due to mainly convenience my shorter range riffles including .22 RF are mainly zeroed at 60 metres and the longer range ones at 172 or 200 metres. It may be worth contacting the Ruahine Deerstalkers regarding longer range shooting. You can find contact details for them on the New Zealand Deerstalkers Association web site.
Regards Grandpamac.

All good points i would say except for the zeroing at 300 to offset spin drift. They do that because the max 'cone of error' of the average soldier matches the killzone on an adult chest. I I can recall the instructors telling me their idea on spin drift in the infantry, but check it out on a ballistics calculator. Its basically nothing even at 1km.

Greetings dead is better,
Thanks for your post. I admit that i was a little vague about the extent of drift so will need to read some more about it. Every day you learn something new is a good day.
Regards Grandpamac.

I don't know that I'd say spin drift is basically nothing. For example ( probably not the best ) a .223 cal. 55gr projectile, BC 0.235, MV 2780 fps, zeroed @ 200yds. Using Strelok Pro the results at 1000 yds: Spin Drift is 19.8"/50.29cm, Coriolis Effect is 4"/10.16 cm. Total of 23.8"/60.45 cm. Certainly enough to worry about at the range, let alone if you were keen enough to attempt a shot on a live target @ 1000+yds.
Just my musings after a couple of beers,cheers.

A very sedate speed for a 223 of that weight.

Greetings All,
Assuming that you have a right twist barrel Woods223 the Coriolis Effect works counter to Gyroscopic Drift in the Southern Hemisphere so should be subtracted from rather than added to Drift.
For you Tararua Phil please remember that, in spite of your thread has gone, if you can find a place to zero your rifles at 100 m for the 7.62 x 39 and 150 to 200 m for the .308 all will be good.
Happy Hunting.

Thanks guys for all your input & advice. ...appreciated.
I'l just sight in 20mm high @ the 50mtre mark for both cals knowing it'll then be on @ about 200 - 300mtres (estimated on sight). An inch lower @ POA for the x39 won't make any difference really as it'll still be in the kill zone & I rarely shoot beyond that anyway due to the bush edge environment I hunt in. The x39 I mainly use on goats & pigs with an occasional red or fallow foolish enough to step into view so both cals'll do the trick if I place the pill in the right spot.
Regards,
Phil.