Thread: Calculating velocity

Hello, I'm new to reloading, but I have worked up a new load for my 6.5x55 using 140gr sst's. Groups are under 22mm consistently.

Is there any way to calculate the velocity from knowing 2 different points of impact at 2 different ranges. I think it is easy if I had my rifle zero at 100 but I didn't think to do that at the time. So the data I have is at 100y my impact was 4 inches above point of aim, and at 165y it was 5.5 inches above. Rifle is ruger with 22inch barrel bullet is hornady sst bc .520.

I have been banging numbers into a ballistics calculator for ages trying to get the points of impact to line up and the closest I can get is a velocity of 2700fps sighted in at 291y gives me a 100y point of impact at 4.45 inches above and 165y point of impact at 5.46 which is pretty close. Is there an app or programe you can enter 2 points of impact without knowing your zero the one I have been using our need your zero range.

Thanks if any one could check this and let me know if I'm on the correct track would be much appreciated.

Originally Posted by rusl

Hello, I'm new to reloading, but I have worked up a new load for my 6.5x55 using 140gr sst's. Groups are under 22mm consistently.

Is there any way to calculate the velocity from knowing 2 different points of impact at 2 different ranges. I think it is easy if I had my rifle zero at 100 but I didn't think to do that at the time. So the data I have is at 100y my impact was 4 inches above point of aim, and at 165y it was 5.5 inches above. Rifle is ruger with 22inch barrel bullet is hornady sst bc .520.

I have been banging numbers into a ballistics calculator for ages trying to get the points of impact to line up and the closest I can get is a velocity of 2700fps sighted in at 291y gives me a 100y point of impact at 4.45 inches above and 165y point of impact at 5.46 which is pretty close. Is there an app or programe you can enter 2 points of impact without knowing your zero the one I have been using our need your zero range.

Thanks if any one could check this and let me know if I'm on the correct track would be much appreciated.

Ideally your two ranges should be further apart to be more accurate.

Try JBM ballistics, set your sight in height at 4''...your scope height will need to inputted accurately also as due to the closeness of the ranges.

As above, max distance between your two ranges, and accurate measurement of your scope height. JBM is pretty simple to use, but it looks like you are about on the mark with 2700. Where are you based, I have a chrony you can borrow if you are in Canterbury.

Thanks for the input guys, I agree further out would be ideal, that's as far as we could get yesterday, I have decided to go somewhere else this afternoon and have another play, I will try jbm. Thanks for the crony offer but I'm way up the top in whangarei so not the most handy cheers though.

Hi Rusl,

From a practical point of view, it's very good to shoot a trajectory on paper then trial and error in a BC and MV that you can enter into a ballistic program to match the actual trajectory you have. Some phone ballistic apps can adjust your MV using the point of impact at different ranges, just like you are trying to do. The main uncertainty is locating your group centre so its great you're getting reasonably tight groups. The air pressure (mainly affected by altitude above sea level) and temperature will make a difference so try and estimate them on the day and include in the ballistics calculator when you do it; check whether you're using degrees celsius and mBar or inches Hg. Some phones have a barometer in them but if you don't have one and aren't sure how to get an approx air pressure, post a question on here.

You really need your further range to be beyond the max you ever want to shoot, so your program is putting in points between ones you've verified on paper. Not extending beyond what you've ever shot on target. So you really need to shoot at 500 yd or more to set up your trajectory (not so easy to find somewhere to do it ...) There are posts on here asking where to find a long public range so have a search.

I once had a bit of a play tossing up whether to adjust muzzle velocity or ballistic coefficient (also known to be subjext to marketing hype) or both: http://www.nzhuntingandshooting.co.n...jectory-21928/. Not sure if that will help or not.

You will probably find that under 200 yd a MV difference of 100fps won't really show up unless you can shoot quarter minute groups.

Finally, if you do try it out at longer range, shoot two groups at the long range. Start by zeroing as well as you can at 100yd and range finder the exact distance. First group at long range like 500 yd with sights set to zero (eg 100 yd) using a very tall target carefully checked it's vertical and measure how far low the shots are in cm or inches. Then adjust your sights up till you get shots hitting where you aim and record how many MOA that took (or else aim high using your reticle and record where the point of impact is on your reticle.) This time the shots should be landing right on your target. If you want to adjust sights or use a holdover reticle for longer ranges then you need to know your true scope adjustments or reticle size, whether they are MOA, inches per hundred yards or whatever, they may not be exactly what the manufacturer says.