308 muzzle velocity question

T

TrueBlue711

New member
The question is more specifically about my SCAR-17. I'm getting my CDS dial on my scope custom made for the ammo I want to have it zeroed in to. I'm using Hornady A-Max 168 grain, which has a muzzle velocity of 2700 fps out of a 24" barrel (per the box). The SCAR-17 has a 16" barrel. Best option would be to use a chronograph myself, but I don't have access to one. Has anybody shot that ammo out of a 16" barrel and chronographed it? I'm being told you lose about 25 fps per inch lost, which would be new muzzle velocity 2500 fps. I just want to know if that's pretty accurate from people's experience. If so, I'll go ahead and order my new dial using 2500 fps. Thanks!
 
I just looked at some of my load data using 165gn Noslers in my 16" AR and 2500fps is spot on.
 
Google "Ballistics by the inch." There is a website that has tested various rounds by progressively cutting down a barrel and chronographing the results. They should have something reasonably close to your load.
 
I'm getting my CDS dial on my scope custom made for the ammo I want to have it zeroed in to
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Nobody else's test can tell you exactly what you are going to get out of YOUR rifle. Might be close, should be close, but the only way to know for certain is to measure what you shoot out of your rifle.

Is 20fps something that matters? is 50fps? 100fps can, if you are having a custom made dial for your scope, best it be made for what your gun and ammo is actually doing, NOT for what anyone's test says it ought to do.

Chronographs are pretty cheap these days, the one I bought decades ago showed me that the same ammo can vary in velocity from different guns, with equal barrel lengths. 100fps difference is rare, but it can happen, I've seen it.

100fps is about 1/2" difference in drop with a .30 bullet, per hundred yards. Roughly. Lots of variables.

How accurate do you need to be? Get your dial cut for what the book says the speed should be, and if your rifle is faster or slower (and it could be either one) with your chosen ammo, then your dial, and your shooting is going to be off, by a proportionate amount.
 
In playing with the numbers using 43.5 gr of 4895, 168 A-Max, I get 2713 FPS from a 24 inch barrel.

Shooting the same load out of a 16 in barrel I get 2473 fps.


Of course this all changes when you switch components. You want to find a powder that burns completely in a 16 in barrel instead of burning after the bullet leaves the muzzle.

Doing that, you have to watch pressure.

The main thing to consider is "velocity doesn't mean accuracy". According to Gen. Hatcher, in developing loads for International 300 Meter Matches, they found that they got their best accuracy at 2200 fps.

I wouldn't lose a lot of sleep over the velocity loss of going to 16 inches. I would concern my self with the accuracy.

I've been shooting the 308 in competition for about 40 years. I keep my velocity at about 2550-2600 fps. With 168 A-Max and 168 SMKs.

That is a comformise between accuracy and working the action on my M14/M1As.

As a side note, I also try to use those same velocity numbers with 175 SMKs for 1000 yard NRA Matches and Precision Rifle (normally the max range is 1400 yards.

I'm also not a fan of Scope Dials for a given round. If you always shoot in the same conditions they are fine, but they don't work sighting your rifle in at 40 degrees and then shooting it at 80 degrees. Or sighting in at sea level and shooting it at 5000 Ft.

I'm of the opinion that the best option is a little note book to record your sight settings for different environmental conditions.
 
The best way to do this correctly is to field verify the dope/ trajectory information that any ballistics program or computer spits out.

Punch the numbers you have- (muzzle velocity, BC, temp, etc) into any free ballistics program like Strelok or JBM- then get out to the range, set up targets and shoot to confirm.

Never take the numbers that are just spit out as the gospel- especially if you're spending bux on BDC turrets and they could be wrong. I know this is commonly done with well known loads/reticles as well- but it's far better to get it spot on, rather than close.

Kraig makes good points, "corrections" are sometimes part of more "sophisticated" turrets- and can also simply be recorded in the dope book for things like temp, humidity, and elevation.
 
tobnpr said:
The best way to do this correctly is to field verify the dope/ trajectory information that any ballistics program or computer spits out.



Punch the numbers you have- (muzzle velocity, BC, temp, etc) into any free ballistics program like Strelok or JBM- then get out to the range, set up targets and shoot to confirm.



Never take the numbers that are just spit out as the gospel- especially if you're spending bux on BDC turrets and they could be wrong. I know this is commonly done with well known loads/reticles as well- but it's far better to get it spot on, rather than close.



Kraig makes good points, "corrections" are sometimes part of more "sophisticated" turrets- and can also simply be recorded in the dope book for things like temp, humidity, and elevation.
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Ditto on this. Ballistic calculators get you close but aren't the end all solution. I wouldn't spend a dime on custom caps until I confirmed my dope in the field.


Sent from my iPhone using Tapatalk
 
No chronograph, but you do have a range you shoot at? How far can you go?

From the same load and gun, I have seen as much as 40 fps difference, so I would do more than guess based on someone else's load and barrel, especially since you are shooting a gasser.

Start with 2500 fps and a 100 yard zero, then take it out as far as you can and figure out what the ACTUAL drop is for your bullet. Using good ballistics software, you can true your velocity until it matches up. Even with a chrono, I would do that if I was going to have a custom turret made.
 
Buy or borrow a Chronograph.

With barrels between about 20"-24" I've found 15-20 fps difference in velocity per inch is about what to expect from a 308. With barrels longer than 24" the differences are much smaller, often in single digits. Once you get below 20" however you do start seeing larger differences as barrel length changes.

Then you have to deal with the differences between individual barrels. I've seen 2 different barrels of the same length shoot 130 fps different. That is an extreme number, but 25-50 fps difference between barrels of the same length is common.

If you're going to the trouble and expense of a CDS for your scope you really need to know the speeds you're getting from your rifle.
 
Thanks for all the info! I'm not doing competition shooting or really shooting at anything over 300-400 yards with this gun. It's more for hunting and SHTF. I also forgot to mention that another road block I have is I'm currently stationed in California...where guns such as my SCAR-17 are at risk if I take them out to public ranges. The only time I get to shoot it and other similar guns of mine are when I go back to my home state to visit family. On that note, my dad has a chronograph. I'm not a very patient person, but I'll wait until my next visit.
 
Heck, to 300 or 400 yards, I would not bother. I might go with a lighter bullet and more velocity.

I run my .308 AR pattern rifle with 140s most of the time, and 155s for stuff out 400 yards plus. With a 200 yard zero, my 155 Amax is down 8" 300 and 24" at 400. The 140s are 1 and 2 inches different as are the 168s. There just is not that much difference in that close unless you are talking precision.

I can mix loads in a mag and still shoot a 5" 400 yard group. With only one, my groups are in the 3" range. I'd spend that money on ammo instead of a custom turret.
 
Chronographs are relatively cheap vs. 30-40 years ago. Without a chrono, you have no idea what your velocities are doing.
 
OP doesn't handload, so IMO a Chrono wouldn't be of much use beyond that first string to establish MV out of the box. For a handloader, a must have- but that's not his case. MV can also be closely estimated without a chrono simply by using the bullet drop dope at long range. You know how much drop you have (and the other relevant inputs) and you know approximately what the MV was

In fact, his actual MV isn't even relevant if he's going to be shooting the same ammo; it's the trajectory (drop) beyond his zero that's needed and that's easily established with targets spaced out to his maximum intended range. A few paper targets is a lot less expensive than a chrono- reasonably priced as they are.
 
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