Muzzle velocity estimate

[sblanch]

Can MV be reliably estimated using load data and bullet drop?

My specs are:
308 win
26" 1 in 10 barrel
178 amax
42.3 grains 8208xbr
750' above sea level
80 degrees F
75% humidity
30" Hg
100 Yd. zero
8.5 MOA drop at 500 Yards

I don't have a chrono and would like to know if this load will still be supersonic at 1000
Thanks

 

[dpreston]

Re: Muzzle velocity estimate

I didn't run your numbers, but I would bet you are good out to 1100 like the 175 smks are.

 

[ewoaf]

Re: Muzzle velocity estimate

actually yes, but you have to shoot it. Just shoot over several ranges out to 1000 then come home and get on JBM. enter your enviro for the day and play with the MV until it gives you your real world dope. Not only do you now have a pretty accurate MV, but you have verified your real world dope. make sure you select the (litz) bullet from the library if there is one for your bullet.

 

[bohem]

Re: Muzzle velocity estimate

I messed with it through JBM and you're in the 2940fps range. My initial reaction is that you have a bad read on your drop to 500 yards, I've yet to see ANY 308 that can push a 175-180gr bullet anywhere near that fast.

 

[dpreston]

Re: Muzzle velocity estimate

damn! That's the same speed I push my 155 scenars!!!

 

[sblanch]

Re: Muzzle velocity estimate

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bohem</div><div class="ubbcode-body">I messed with it through JBM and you're in the 2940fps range. My initial reaction is that you have a bad read on your drop to 500 yards, I've yet to see ANY 308 that can push a 175-180gr bullet anywhere near that fast.

</div></div>

I used JBM and none of the real world drops between 100 and 500 yards made any sense unless I jumped the MV to near 3000 fps, which seemed extremely high to me. Maybe the dials on my scope aren't correct?

 

[Grump]

Re: Muzzle velocity estimate

When I ran JBM (with a 2.5 inch sight height, which I don't think makes a big difference past 300...), 2900 even gave 8.5 MOA to 500.

I don't think even the new Hornady Superformance stuff with the other 178-grain bullet goes that fast from a 26-inch barrel.

 

[Grump]

Re: Muzzle velocity estimate

I'd confirm your range first. The more common dial errors are .25 IPHY (less correction=MORE clicks needed) as compared to 1.04 or so inches for 4 clicks, 1 "MOA".

At a slightly more sane 2800 fps, the true MOA correction just happens to be 8.5, for 475 yards. That's a shortage error of 5% from the 500 that your range (existing layout? or optical ranging like mil-dot? or laser?) is supposed to be.

Like a lot of other algebra/calculus problems, the accuracy of your initial numbers for this has a HUGE and unavoidable effect on the accuracy of your calculated result.

 

[BCP]

Re: Muzzle velocity estimate

That load should be around 2600-2650 according to the reloading manuals I'm looking at. I don't know where your 8.5 MOA is coming from, are you zeroed high at 100? I really don't think you're getting 2900fps (which is like a 300 win mag) from 42 grains of powder.

 

[former naval person]

Re: Muzzle velocity estimate

As BCP says...check your 100 yard zero accuracy. Do a 10 shot group at 100 to establish the center. A half inch off there can put you WAY off at distance. Also, check your ranges. We shot for years at a 400 yard range and found out last year it is 382 yards. If you can find a properly surveyed range instead of a lasered range, you'll be able to increase your confidence level. JMHO

 

[sblanch]

Re: Muzzle velocity estimate

My range is set up in a pasture with targets at 100 yd increments. We wheeled the distance over rolling ground so an error in distance wouldn't be out of the question but I would be suprised if it was off by 25 yards. If lasers aren't reliable enough, whats a good way to get an accurate range?

 

[BCP]

Re: Muzzle velocity estimate

Lasers are reliable. Buy a chrono they are worth it.

 

[sblanch]

Re: Muzzle velocity estimate

I got my hands on a Bushnell Elite 1500 and checked the distances on my range and found that the 500 yard target was 495 and my 100 yard zero was 98.

I used this info with JBM and the velocity still has to be over 2900 to make my drops make sense.

I did check my zero with a 10 shot string and it is right where it should be.

Is it completely out of the question that my velocity may be that high?

 

[BCP]

Re: Muzzle velocity estimate

Buy, borrow, or steal a chrono. It's the only way to know for sure.

 

[bohem]

Re: Muzzle velocity estimate

I was chided on another forum for trying to correlate velocity with changes in drop. I'm not convinced yet that "barrel whip" is the only factor involved to take up the difference that I saw in a friend's rifle, but it is a possibility for consideration.

The best thing would be to beg/borrow/buy a chrono.

 

[sblanch]

Re: Muzzle velocity estimate

I got my hands on a chrono today and shot 15 rouds over it.

I came up with an average of 2700.6 fps. These were fired at a lasered 100 yards which confirmed my zero as being good.

Plugged that into jbm and it called for 11 moa at 500yds. On my Leupold Rifleman (stifle the laughter please) I dial about 8.5 moa for that range. I say about because the Rifleman uses friction dials and a rotatable ring/washer with 1/2 moa hash marks on it so being precise is difficult.

I went ahead and checked my scope at 100 with a yard stick and it seems that my scope raised the reticle about 1.26 moa instead of the 1 moa indicated on the turret. Did the math and suddenly all of my data from the range and JBM makes sense.

I guess I'll live with my 1.26 moa adjustments until I can afford a PST, unless anyone thinks it is worth bringing to Leupold's attention.

Thanks for all the responses guys.

 

[sobrbiker883]

Re: Muzzle velocity estimate

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 5x5GUY</div><div class="ubbcode-body"> Maybe the dials on my scope aren't correct? </div></div>

I'll bet if you mentioned the make/model of scope priviously more folks would have agreed with that assumption off the bat (I would have if I'd noticed this earlier).

Now you can plug "1.26 moa" into the "units" on jbm and make a chart for that scope's marking-that will keep you going until you get a better suited scope. At that time I'd send it to Leupy and see what they say.