7.62 NATO load question

[edward]

Figured this was the more apt forum to ask this question in than the Centerfire Rifles section.

I am looking at an optic. This optic has a Bullet Drop Compensator built into the reticle. The BDC is calibrated for 7.62 NATO M80 ball ammunition, traveling at 2810 FPS. According to the manufacturer, this is without regard to barrel length; they took a mean value of 2810 FPS muzzle velocity and developed the BDC accordingly for range estimation given the projectile weight and muzzle velocity.

What I am trying to find out is this; if one had a round of M80 ball ammunition, what length barrel would it need to be fired out of to achieve that 2810 FPS muzzle velocity, in order to accurately make use of the BDC? Could it reach that threshold from a 16" barrel? How about an 18"? 20"? 24"?

Load data for M80 ball as follows;
Bullet Diameter: 0.308
Powder: WC 846
Charge wegith: 46 gr.
Chamber Pressure: 50,000 PSI
Projectile Length: 1.14"
Cartridge case capacity: 56 Gr.

I realize I am trying to figure this out in a somewhat backwards way, but being that I don't handload or have a chronograph I need to figure out the answer to this question using straight math.

Thanks for any help.

[Jonathan Doe]

I have tested Pride-Fowler scope with the BDC reticle to 800 yards. It was accurate for Federal Gold Medal 168 grain HPBT ammo. Once, I sighted the rfiel at 100 yards, I was able to hit targets at known distances with first shot at each distance.

I also used a Leupold Mark 4 M1 scope with a custom turrett made by a company in Moorpark, CA. It was pretty accurate. It was calibrated for 168 grain bullets at 2,700 fps, at 1,000 ft elevation and 75 degree temperature. The turrett itself cost me $65.

[Gasman]

Your MV (and hence the resulting trajectory) will depend on your barrel and chamber, so you cannot accurately predict what your MV will be just based on a listing of the load components and the length of the barrel. No two barrels of the same length will shoot exactly the same, and in some instances, a shorter barrel may produce a higher MV than a longer one.

The upside of a BDC reticle is that it's fast to use. The downside of a BDC reticle is that it is spot on only for one particular combination of MV, BC, elevation, temperature, etc. Otherwise, it will be off to some degree, but depending on what distances you're shooting and how precise you need to be, it may be good enough. If not, turrets are a better solution for you.

In other words, there are no good answers to your question.

[edward]

This is not a precision application. +/- 30 fps is well within acceptable standards, I am not lookinf for down to the last tenth of a degree calculations.

Given that, there has to be someone with the requisite knowledge to be able to answer this question within the ballpark.

The 2810 fps mv is a generic published velocity for m80 ball, as I have come to find out in the last few hours. It is something the military lists in all of the TMs I have read. The trick is just figuring out what length test barrels they blast m80 ball out of to come up with these figures.

[Gasman]

You're not getting it - MV is not something that is calculated; it is something that is measured. You cannot dump the powder out of a case of Federal Gold Medal Match, measure the weight of the powder, and then calculate the MV of the load by some mystical formula. No, what you do is to take the rifle out to the range with a chronograph, fire it, and measure the MV. Then, if the factory's BC calculations are correct (a big assumption), and if you are at the correct elevation/temperature/humidity with the bore of your scope precisely at 1.5" over the bore of the rifle, well then maybe the trajectory of your round will correlate with the mythical one used to design the BDC reticle that you're interested in.

Note: BTW, depending on the powder used, the MV can vary by a whole lot more than 30 fps depending on the ambient temperature. So, assuming that the MV of a load is a constant is also a mistake.

[Gasman]

I redact all that I wrote above.

The correct answer is 22".

[Trevor]

Ignore Gasman, he's an eh hole... The correct answer in 18"

[Gasman]

21.8736295"

That rounds up to 22".

[Gasman]

Actually, YMMV, so I would start with a 24" or 26" barrel, and if the MV exceeds 2810 fps, keep cutting off an inch or two at a time until you reach the desired velocity. Don't forget to crown the barrel afterwards.

[edward]

Thanks for the verification on 22". I was able to find a published source yesterday that had M80 ball out of a 24" test barrel slated at around 2855 FPS. Plugged in a 22" barrel using all the same data to an online calculator and velocity dropped to 2810ish.

FWIW; my rifle is an 18" barreled PTR-91. I needed this information to see if a particular optic (a Trijicon TA648-308) would have a BDC that is even in the useful ballpark for what my application is. Knowing what length barrel I would theoretically need to use to achieve the correct muzzle velocity with mil-std ammunition to make use of the BDC helps figure out how far out of the ballpark the BDC will be if I try to use this optic on my shorter barreled rifle with the same ammunition. In that respect, it is indeed somewhat out of the realm of immediate usefulness.

[Gasman]

All you had to do in the first place was ask if an ACOG with a 7.62x51 reticle would work on a rifle with an 18" barrel.

In the first place, all M80 ammo is not the same. All of the various manufacturers, and even different lots from the same manufacturer, will produce different results from the same rifle with respect to both MV and accuracy. This is oftentimes why some brands of M80 are more desirable (and hence more expensive) than others.

Secondly, how well do you expect an H&K clone to shoot using M80 ball ammo? I think 3 MOA is a reasonable expectation. 3 MOA is far more than the difference between the trajectories of M80, Federal 168 grain Gold Medal Match, M193, and even 223 69 grain BTHP ammo at say, 500 yards - MUCH more. The actual difference between those loads is actually closer to 1 MOA at that range. In other words, your rifle is not accurate enough to take advantage of a BDC reticle at longer ranges and your concern about if the reticle is close enough to the trajectory of your load given a barrel length shorter than that of the test barrel is a very minor issue.

Then there are other issues. What is the size of your intended target? What is your acceptable difference between POA and POI? How far are you planning on shooting? How well can YOU shoot at that range? How precisely are you able to range targets in the field? These are all important variables.

Throwing an optic that is capable of shooting at range does not mean you will be able to shoot at range. Throwing money at a problem does not necessarily create a solution. Looking at ballistics tables online does not solve the problem. An understanding of the various factors involved and the skills to utilize the tools that you possess ARE the solution to the problem.

[Gasman]

And jesus, if you can afford to buy a $1000 rifle and a $1000+ optic, get a friggin' chronograph. Do the basics first, then get the fancy stuff.

[edward]

You're far over-thinking this. The simple questions have been answered, and I have satisfactorily gathered the information I have been looking for.

This information gathering exercise served the purpose of letting me see theoretical numbers before I even spend a single red cent of my money on either the optic, the ammo, a chrono, or a hand-loading setup.

I wouldn't set about building a 1000HP stock car without reading books about it first, and I wouldn't buy a $2500 optic without finding and reading the data that backs the supposed design of that optic.

[Gasman]

Try shooting a little bit. That would help.

[edward]

Ahhhh summer days are here again.