.308 ballistics question

[HRnightmare]

Recently I shot my .308 AR10 with Rainier Amrs ultra match grade 20" barrel and BCG. I was shooting 174 grain hollow point folktales and with my scope zeroed at a hundred yards I was hitting about six inches low at 200 yards. From several other ballistic calculator stand for on the most people seem to think I should be between two-and-a-half to three inches.

The range I shot it only goes out to 200 yards but eventually I would like to qualify to shoot at the 600-yard range but if I'm dropping 6 inches with only 100-yard difference I'm afraid that I'm not going to be able to even get on paper at 600

 

[eldon519]

Perhaps a dumb question but it was zeroed at 100yds with the same ammo or a different load? Are these handloads?

 

[ontarget]

Also, there is a big difference between 600 yard drop if you were to zero at 200 instead of 100. And most ballistics calculators are a good guess at best.

 

[Robert101]

I wouldn't be so concerned about the ammo shooting 6" low. That is I assume this is not the ammo you zeroed your rifle to in the beginning - right? If so, something is off. Re-zero this ammo at 100 yards and then you should be close out to 600 yards. I would suggest that you shoot some intermediate targets say 200, 400, and then go out to 600 yards to confirm the initial testing.

 

[FL-NC]

Key components of the puzzle are the muzzle velocity that YOUR rifle is getting with THAT round. Also, the size of the shot group at both distances, and an honest answer regarding the shooter's ability to shoot the group that the rifle/ammo combo is capable of.

 

[Art Eatman]

Round numbers: Zeroed via 2" high at 100 is pretty much dead-on at 200. Around six to seven inches low at 300. Roughly two feet low at 400, and maybe a tad over four feet low at 500.

600? I'd have to look it up.

 

[jmr40]

A good bullet with a BC of around .5 leaving the muzzle at around 2650 fps will be about 4" low at 200 yards if zeroed at 100 yards. That is what I'd get with a 180 gr Accubond from my 308.

I have no idea what the BC is on that bullet or how fast it is leaving YOUR barrel. But if your substantially slower than 2650, and/or that bullet has a poor BC in the low .4's then 6" low at 200 yards is very much possible.

You need to determine the exact BC of the bullet you're firing and the exact speed it is leaving YOUR rifle with a chronograph. You can then plug the numbers into one of the many online ballistics programs. It will calculate your drops out to 2000 yards depending on the program you are using.

Once you know the drops, and the exact range it is possible to zero the scope for that range. With good optics you should be able to get to 600 even with 6" of drop at 200.

 

[HRnightmare]

The scope is zeroed and dead on at 100yds. It is the identical ammo used to zero the rifle and the only ammo I have shot out of it thus far.

at 100 yds I put 12 rds all within the center circle of the target which is about 2.5". the circumference of a soda can covered all but the very outside edge of the shots

at 200 yds , once I figured out where POI was vs POA and held high 6" I put 12 shots within an area just about a 4" circumference which is slightly smaller than the circumference of a 1GAL bleach jug. I shot two rounds and they were literally touching but about 6" low. I took an identical target and measured about where they were to about where the bullseye was, aimed that distance higher and was right back to dead center.

I proceeded to put about 25-30 more shots in similar sized targets at the same distances, maybe increasing my groups slightly as I got tired. Also towards the end I was firing much more rapid to test my ability.

I am by now means Chris Kyle now BUT considering this is my first time shooting this rifle at that distance and was able to consistently stay within those groups bench rested supporting the rear end of the rifle with my free hand I think the issue is not my ability. I am very new to longer range shooting so I would consider myself exactly "entry level". I know I can improve my groups with a lighter trigger, better support on the rear end and shooting from something other than a bench but for this I do not believe I am a huge factor. Because as I said the shots were consistently low, and then when I corrected consistently on.

I guess my biggest surprise is the distance dial on my Vortex VIPR PST was once again dead on when at 100yds. I cranked it to 200 yds and it was still 6" off. Essentially rendering that distance adjustment useless, inoperable on my scope OR most likely not being utilized properly.

FWIW I was shooting 175GR HPBT from Freedom munitions. I chose these specifically because I read on several forums this caliber was an excellent hunting round. I do not intend to hunt with this rifle but like the idea of having something that can hit hard far out there and am wanting to learn how to do so.

WORST CASE SCENARIO - I go to the "course" to gain access to the 600 yd range which I believe has 100 or 50yd increments and take a few shots to figure out where the sweet spot is at each distance.

 

[HRnightmare]

honest answer regarding the shooter's ability to shoot the group that the rifle/ammo combo is capable of.

Actually I am glad you mentioned this because I am curious as well. All I know is that the other guys on the long distance sight in range who at the sound and look of it had been doing this for a while and certainly knew how to "talk the talk" and use the proper terminology were either being polite or seemed at least slightly impressed at my targets.

Given the groupings I described: (soda can circumference grouping at 100yds and 1gal bleach jug circumference at 200yds) what would you say about them? Ok, great, terrible? (I have thick skin). I just have no frame of reference to compare to.

I realize after leaving I did not do so many things I should have. Instead I took an identical target and measured approx. distance with my hand and held high on my next shots. Obviously if I find myself in a tree blind I cannot accurately measure what 6" is as accurate when it is an unfamiliar target. I did not attempt to make adjustments to the scope to see if I could adjust it to be dead on at 200yds because I was nervous and afraid I would not be able to re-zero to 100. I also did not take note of exactly where when aiming high my crosshairs met. Nerves and not thinking on anything other than :should this be happening" got the best of me.

 

[FL-NC]

If you are shooting EXACTLY 2.5" group(s) consistently @ 100 then you and that gun are a 2.5 MOA system. All things remaining equal, a 5" group or better is predictable @200 (5 MOA). It sounds like you are shooting some type of factory reload. I don't know anything about that round, the QC in manufacturing, etc., nor can I speak to the quality of your scope, how properly its mounted, or the MV your gun is putting out. I would try some 168 grain Fed. gold medal match, and see what happens. If I had to guess, I would say the ammo may be a weak link.

 

[HRnightmare]

it is Freedom Munition reloads. The scope is a Vortex VIPR PST 6-24 X 50 mounted on a BURRIS PEPR. I guess I should have also mentioned I am shooting it suppressed with a YHM Phantom .30 suppressor.

 

[eldon519]

Did you bu chance zero it without the suppressor and then shoot this group with it? Suppressors can cause some change in POI.

 

[HRnightmare]

No, zeroed and only shot with the suppressor. The groups are not really my concern. I know with practice a monopod or rear rest and lighter trigger I will tighten those up. I just didn't expect such a drop at such a little difference in distance.

 

[jmr40]

You are shooting cheap reloads, plinking ammo. I went to their website and they show 2620 fps with a .505 BC on that bullet. There is a very real possibility they are generous on the 2620 fps, it may well be 100 fps slower than that. Combine that with a shorter 20" barrel and you can take off another 70-100 fps. There is a very real chance you are only getting 2400 fps from your rifle and 6" low is right where it should be.

But even you were getting 2700 fps you'd be 4" low with that bullet. Even with the load you have you'll be about 9-10' low at 600 yards. While that sounds bad, even a 300 WM is going to be 2.5" low at 200 yards and about 6' low at 600 yards with a 100 yard zero. You aren't as far off as you think.

Your bigger problem is a 2.5 MOA rifle. Hopefully better ammo will shrink group size.

 

[wbstx11]

Try again with a good commercial ammo. I recently tested some various 308 reloads of mine, and also tested some Hornady, Fiocchi, and Freedom Munitions remans at the same time for comparison. The FM were all over the place with a 5" group at 100 yards. All the other ammo had a 2" group at worst. I suspect the variety of cases and an inconsistent powder charge is a contributor. BTW, I was shooting a Ruger American with a Leupold VX-1 2-7.

 

[Jackal]

Freedom Munition's loads pretty darn good ammo. Its generally more consistent than bulk FMJ ammo, though I've never really had "match" results with it. I have however had incredible luck with PRVI 168 match load.

 

[HRnightmare]

Freedom Munition's loads pretty darn good ammo

that was the opinion I got from many on several forums that for precision, this was a very good caliber and a very good load of bullet for "long range hunting" which I assumed you need to be most accurate while shooting a moving target.

Perhaps I'll try the 168gr. The rifle barrel / BCG to be sub MOA at 100yds. I am awkwardly supporting a very heavy rifle with my weak arm and am brand new to this and not able to go prone which I think would make me better. I truly believe the rifle if I were to sled it would easily be within 1MOA. Right now my concern really isn't about my groups, its just about the unexpected amount of drop.

I guess the short answer is I need to get the actual FPS out of my barrel to get a realistic idea of what the ammo is actually performing.

 

[rbernie]

Inside of 300 yards, I would expect most 308 loads to be more velocity sensitive than bullet BC sensitive. In other words, moving to a lighter/faster load (zero'ed at 100 yards) would decrease the drop at 200 yards but increase the drop at 600 yards. Conversely, using a heavier/slower/slipperier load will help at 600 yards but not shoot as 'flat' closer in.

So - pick your load based on the distances at which you want most to shoot.

 

[3212]

I vote for a lack of velocity as the problem.Do you know any body with a chronograph?

 

[Llama Bob]

I second the chronograph. The trajectories from ballistic calculators can be very accurate, but only if everything matches your actual conditions. The parameters that are most frequently wrong in my experience are
- elevation/temperature/air density
- bullet velocity
- sight height above bore
- ballistic coefficients that don't really match the ammo (they're for the wrong velocity, single G1 #s on a bullet shaped like a G7 projectile etc.)
- actual distance to the target
- shooting at a down/up angle

In terms of calculators, I would put a fair amount trust in JBM ballistic using the Litz measured G7 coefficients for boat tail bullets unless you're shooting past 1000y.

http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi

I would start with measuring the velocity of your ammo. Although I admit what's going on doesn't make much sense to me as the velocity needed to get a 6" drop between 100y and 200y is pretty low. I think your drop with a 2.5" sight height above bore at sealevel should be in the vicinity of 3-4" depending on real velocity. 6" is a lot.

 

[MistWolf]

The rifle in question is an AR. That means it has a taller than normal height over bore for the sighting system. The taller the height over bore of the sights, the less the bullet will rise over line of sight for any given distance. The result is that more of the trajectory arc falls below line of sight, thus seeming to give more drop at longer ranges.

With most 5.56 loads, a zero of 100 yards with an AR means the bullet will intersect line of sight at 100 yards and never rise above it. But, if the height over bore of the sights is lowered significantly, the trajectory of the bullet, when zeroed for 100 yards, will cross the line of sight twice- first as the trajectory takes the bullet above line of sight and again as the bullet drops the intersect like of sight at 100 yards.

The geometry of this means that the bullets will drop below line of sight more the more height over bore your sights have. This means the 6 inch drop at 200 is likely to be normal for the height over bore of the sights of an AR

 

[blarby]

it is Freedom Munition reloads.

Absolutely the weakest link in your system.

I would certainly second a chronograph, and I would start with a known match quality factory load. Federal GMM in 168, Hornady Match in 168, there are quite a few.

Once you have a better idea of your true capabilities in that "system" with some proven constants, you can better "grade" your results, and make adjustments to your style or gear.

I wouldn't kick yourself over the results of 2nd tier factory reloads.

We've gotten some really good batches of Freedom, and some really HORRIBLE batches here at the shop. Its hard to tell which ones you got your hands on. :/

I'd use a known standardized new loading before you start making adjustments.

 

[outlawjw]

Plugged you into my calc & I get 2117 fps for your velocity to hit 6" low at 200 sighted at 100

 

[Llama Bob]

Plugged you into my calc & I get 2117 fps for your velocity to hit 6" low at 200 sighted at 100

That's close to what I got. I don't think these loads are likely to be anywhere close to that slow. I suspect it's over 2500.

Something else is quite likely going on.

 

[Haxby]

2400 fps, .4 bc, about 5" low at 200. With a 2.5" group at 100, 5" or 6" low is completely possible.
As far as I know, any 175 gr hpbt is a target bullet, not a hunting bullet.

the distance dial on my Vortex VIPR PST was once again dead on when at 100yds. I cranked it to 200 yds and it was still 6" off.

What dial is that?