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Old September 17, 2021, 02:35 PM   #1
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How accurate are ammo manufacturer's claimed MV numbers?

I don't have access to a chronometer, and I am trying to develop DOPE for my 308 bolt action. I have about 500 rds of SMK loaded ammo (Bullets 1st) which is accurate and affordable. They quoted velocity out of an 18" AR-10 barrel which is difficult to extrapolate to my 26" bolt action barrel.

I am also trying to devise a drop chart for an AR-47 that I mostly feed with Tula.

In your experience, how accurate are ammo manufacturer's claimed MV numbers?

I suppose this vary by manufacturer, and in that case I would appreciate your experience with the factory ammo that you are familiar with.
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Old September 17, 2021, 02:49 PM   #2
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They are very accurate for the barrel, component lots and technique used. The same ammo won't shoot the same speed in other barrels.

Your load will change about 25 fps per inch of the barrel length.
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Old September 17, 2021, 03:10 PM   #3
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Originally Posted by Bart B. View Post
Your load will change about 25 fps per inch of the barrel length.
Thank you, this is useful info.
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Old September 17, 2021, 04:17 PM   #4
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Here's some icing on this conversation cake.

Rarely will two people shooting the same rifle and ammo get the same velocity.
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Old September 17, 2021, 05:15 PM   #5
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There have been countess tests done and data posted on the internet where people have measured velocity, cut the barrel down 1" and repeated until they got to the legal minimum. The velocity difference is usually a lot less than most think, especially with common barrel lengths.

The problem is that even from the same barrel length you will see very different speeds from different barrels. Around 25-50 fps is normal and I've seen some cases where there was 100fps difference between 2 different rifles with barrels the same length.

I have a 308 with a 20" barrel that shoots faster than either of my 22" rifles. And there is 20-30 fps difference between each of the ones with 22" barrels. My 22" Winchester 30-06 is consistently 60-90 fps faster than my 22" Remington 30-06.

Even if you were working with load data for a 26" barrel it wouldn't necessarily be accurate for YOUR 26" barrel.

In your case I'd take the speeds from the 18" barrel and add about 100 fps to it and run the numbers. That should be close enough to get you on paper with your shots. You can tweak things from there.
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Old September 18, 2021, 01:10 AM   #6
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I find quality manufacturer's quotes to be very close, sometimes right on, even if I don't know what barrel they used. Hornady's stuff for example.
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Old September 19, 2021, 11:42 AM   #7
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Whoops seem only handguns. oh well
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Old October 26, 2021, 06:57 PM   #8
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Kind of a long story but keeping it short, I had occasion to test 5 lots of 270 Winchester. 130 to 150 grain. I was using a LabRadar.

All were a bit higher for listed velocity. My barrel is 26 inches so that is longer than the old standard of 24 and what seems to be the new standard of 20.
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Old October 27, 2021, 02:52 AM   #9
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Without a chrony, you are guessing.
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Old October 27, 2021, 02:23 PM   #10
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Some manufacturers are pretty close to 'average' with their advertised velocities. Some are way off, because they're advertising their best numbers from a test barrel.
In my experience, Nosler and Federal tend to hype the numbers. And most "hyper" or "ultra" velocity branding is going to be optimistic. Hornady Superformance is one that fits there.

Your load will change about 25 fps per inch of the barrel length.
This "rule of thumb" is never right.
I have never seen this be even remotely correct for barrels that I have cut.

Is it a reasonable baseline for expectations when cutting a barrel?
Maybe. For 'standard length' barrels, shooting certain cartridges.
But it is never right on the money, and has no chance of being realistic if the barrel is unusually long, or starting 'short'.

If you want to know what the muzzle velocity is from your barrel, you need to buy a chronograph or find someone that will let you use theirs.

I have had many experiences like jmr40's.
In one 'outlier' case, I maintained the same velocity from a .270 Win barrel after cutting it from 22 to 19 inches. (Same ammo, before and after. Six loads tested, I believe.)
Don't even try it. It's even worse than the internet would lead you to believe.
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Old January 13, 2022, 06:11 PM   #11
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Late to the party, but thought some might find this useful.

The bigger the volume behind the bullet, the smaller the percent change in volume each additional inch of bullet travel expands the volume behind it (the volume the propellant gas and burning powder occupy). Any rule of thumb will be right at some particular barrel length, but it changes with barrel length, becoming smaller as the barrel you start with gets longer. If it changes 25 fps going from 18 to 19 inches, it might change 15 fps going from 31 to 32 inches.

In this approximating table, 308 will be in the left column, as it is technically a medium power cartridge and not an "overbore" cartridge. Just multiply the published 308 ammunition box velocity by the number in the table for your barrel length to learn what the velocity would be if you had a chamber and bore with the same exact dimensions as the manufacturer's SAAMI P&V (pressure and velocity) test barrel and fired it under the same conditions. 24" is where the multiplying number is 1.00 because 24" is the single most common SAAMI P&V rifle barrel length.

There are a few SAAMI rifle P&V barrels that are exceptions to the 24" standard. These are:

.277 SIG Fury 16"
7.62x39 20"
300 AAC Blackout 16"
.30 Carbine 20"
350 Legend 16"
350 Rem. Mag. 20"
351 Winchester 20"
44 Rem Mag 20" (rifle only)

For those, take the velocity multiplier at your barrel length and divide it by the velocity multiplier for the SAAMI actual test barrel length and use the resulting number as the multiplier for the velocity on the box to estimate your velocity.

In real life, you can figure your velocity will typically be for a looser chamber and will be slower, as others have already spelled out. SAAMI P&V barrels have minimum chambers. If you can't measure velocity, I would generally run the ballistic table for the velocity resulting from that calculation and again for a velocity 100 fps slower and figure your result is most likely to be somewhere between the two. I think you will be pleasantly surprised to find it makes less difference than you might expect. The reason is that when you zero your sights, you are already compensating for the velocity difference at your zeroing range. If, say, your barrel shoots slower than expected, the extra elevation from zeroing at that lower velocity will compensate the point of impact over quite a range.

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Old Today, 01:00 PM   #12
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SAAMI says ammunition tested subsequent to manufacture using equipment and procedures conforming to their guidelines can be expected to produce velocities within a tolerance of ±90 fps of their tabulated values.
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