Thread: Barrel chop

Have just had my 308 barrel chopped from 24 inch to 20 inch to add a suppressor.
It is nicely balanced and a good compact rifle now.
I was a bit disappointed when testing the loads out through the chronograph to see the velocity has dropped 200 fps.
The 150 gn were doing 2820 fps but dropped to 2620 fps.

Was told barrel chopping doesn't effect the 308 as much.
Do I accept the lower velocity or change powder loads?

My 308 at 16.5 inches doing 2540 with 168 eldm.
It still flattens deer at 600 so don't over think it.

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Work out your minimum acceptable impact velocity and energy. Pull out your ballistic calculator and see what range that gives you under hunting conditions. Then you will know if you need to change powder.

why wouldnt you think it would drop 50fps per inch?????

and here is another perfect egzample of why I have no time at all for chronicgraphs
you happy with rifles balance,assumably its shooting good groups still..... so where is the problem????
bottom line if your going to dialup/twiddle n fiddle you need a few more clicks..whoopdeedoo the deer wont care as andy has said,dont over think it.

Originally Posted by yogi

Have just had my 308 barrel chopped from 24 inch to 20 inch to add a suppressor.
It is nicely balanced and a good compact rifle now.
I was a bit disappointed when testing the loads out through the chronograph to see the velocity has dropped 200 fps.
The 150 gn were doing 2820 fps but dropped to 2620 fps.

Was told barrel chopping doesn't effect the 308 as much.
Do I accept the lower velocity or change powder loads?

Greetings @yogi,
My suppressed .308 with a 20 inch barrel chronographs 2,750 fps with my handloads. Your speed at 2,620 seems low so I wonder what your load is. Mine has a 150 grain corelokt ahead of 45 grains of Dupont IMR4064 in a Federal case. I did a bit of testing with different powders a while back and 150 grain Hornady projectiles with 44 grains of AR2206H (under max) gave 2,670 fps in a Winchester case. Hodgdon/ ADI lists a max of 45.5 grains which would add around 75 fps to the velocity. Please share or PM me your load details and I can make some suggestions.
Regards Grandpamac.

Originally Posted by grandpamac

Greetings @yogi,
My suppressed .308 with a 20 inch barrel chronographs 2,750 fps with my handloads. Your speed at 2,620 seems low so I wonder what your load is. Mine has a 150 grain corelokt ahead of 45 grains of Dupont IMR4064 in a Federal case. I did a bit of testing with different powders a while back and 150 grain Hornady projectiles with 44 grains of AR2206H (under max) gave 2,670 fps in a Winchester case. Hodgdon/ ADI lists a max of 45.5 grains which would add around 75 fps to the velocity. Please share or PM me your load details and I can make some suggestions.
Regards Grandpamac.

Loading 159gn sierra gamekings with 45 gns od adi 2208.
I expected velocity to decrease a bit but not 200 fps

Originally Posted by Micky Duck

why wouldnt you think it would drop 50fps per inch?????

and here is another perfect egzample of why I have no time at all for chronicgraphs
you happy with rifles balance,assumably its shooting good groups still..... so where is the problem????
bottom line if your going to dialup/twiddle n fiddle you need a few more clicks..whoopdeedoo the deer wont care as andy has said,dont over think it.

Have always heard the 308 isn't affected as much as some calibres with cutting the barrel back.
Was expecting around 100 fps decrease.
It's still grouping well and won't be shooting past about 300 yards maximum.

Originally Posted by Micky Duck

why wouldnt you think it would drop 50fps per inch?????

and here is another perfect egzample of why I have no time at all for chronicgraphs
you happy with rifles balance,assumably its shooting good groups still..... so where is the problem????
bottom line if your going to dialup/twiddle n fiddle you need a few more clicks..whoopdeedoo the deer wont care as andy has said,dont over think it.

Well it's calibre dependant on the loss of fps plus the drop of say 24" to 20" it will be X amount of FPS per inch and from 20" to say 14.5" it will be X amount of FPS per inch.

But as you have rightly pointed out for this case the deer at 300y will not know the difference.

yeah its always been said it will between 20-50fps.... some folks worry about it,some will drop projectile weight and rev it up somewhat... others just go shoot stuff and be happy. sounds like Yogi has it sorted ,just bit surprised by the amount.sub 300 yards he will be fine...hope he has better luck finding a "something" to shoot than I have as of late

YEP 50fps is about right per inch of barrel. Which makes me wonder about the current obsession with cutting barrel length.

Originally Posted by yogi

Have just had my 308 barrel chopped from 24 inch to 20 inch to add a suppressor.
It is nicely balanced and a good compact rifle now.
I was a bit disappointed when testing the loads out through the chronograph to see the velocity has dropped 200 fps.
The 150 gn were doing 2820 fps but dropped to 2620 fps.

Was told barrel chopping doesn't effect the 308 as much.
Do I accept the lower velocity or change powder loads?

Don’t worry if it’s still grouping. I’ve shortened mine to 18 and am running a light load of 44gn 2208 with 150s. Group at 200 is now tighter than the pre chop chop.

A loss of 200fps going from 24" to 20" doesnt add up with the .308, you should theoretically have lost around 100-120fps at most.

My suggestion would be that theres an error somewhere with either chronograph or loading. 44gr is a pretty light load to be doing 2820fps with a 150gr so may be as simple as a chronograph error, or those loads were hotter than 44gr.

End of the day its still going to do the job perfectly fine at the distances you indicate your going to hunt at, but you are right in thinking a loss of 200fps is abnormal. Velocity loss vs barrel length is not a linear thing, it is a curve that ramps up at the beginning of the barrel and then flattens off as the barrel length increases. Cartridge type / bore size / powder used all effect the shape of this curve and therefore how much velocity is lost when shortening can never be a set "x fps for y inches". Due to the shape of the curve, the overall length plays a big part too. For instance with a .308, going from 28 inches to 24 will lose you just under 100fps, however if you were to shorten a barrel that was 18" to 14", you would lose close to 200fps.

Originally Posted by 6.5 CRD

A loss of 200fps for 4 inches doesnt add up with the .308, you should theoretically have lost around 100-120fps.

My suggestion would be that theres an error somewhere with either chronograph or loading. 44gr is a pretty light load to be doing 2820fps with a 150gr so may be as simple as a chronograph error, or those loads were hotter than 44gr.

End of the day its still going to do the job perfectly fine at the distances you indicate your going to hunt at, but you are right in thinking a loss of 200fps is abnormal. Velocity loss vs barrel length is not a linear thing, it is a curve that ramps up at the beginning of the barrel and then flattens off as the barrel length increases. Cartridge type / bore size / powder used all effect the shape of this curve and therefore how much velocity is lost when shortening can never be a set "x fps for y inches". Due to the shape of the curve, the overall length plays a big part too. For instance with a .308, going from 28 inches to 24 will lose you just under 100fps, however if you were to shorten a barrel that was 18" to 14", you would lose close to 200fps.

If he's using an optical chrono it could be anywhere + or - 100fps depends on lighting conditions....optical chrono's drive me nuts with their inconsistent velocity more than once I've gone out same load different days in different conditions (cloudy vs sunny) and got different velocities due to using an optical chrono, shot same poi just got different velocities due to lighting being brighter or darker. If you don't know someone with a Labradar or a magnetospeed your best option is to shoot at range say 100y an 200y then use drop to calculate your velocity.

Originally Posted by dannyb

If he's using an optical chrono it could be anywhere + or - 100fps depends on lighting conditions....optical chrono's drive me nuts with their inconsistent velocity more than once I've gone out same load different days in different conditions (cloudy vs sunny) and got different velocities due to using am optical chrono, shot same poi just got different velocities due to lighting being brighter or darker. If you don't know someone with a Labradar or a magnetospeed your best option is to shoot at range say 100y an 200y then use drop to calculate your velocity.

Yeah your not wrong, Optical chronos can be a pain in the ass.

Calculating velocity from shot drops can work but its not a very accurate way of doing things imo. Alot of variables & not only that, need to shoot at some serious distance to try filter out the noise. 200y just isnt enough distance to give you an accurate idea of velocity. Difference in drop with the 150gn Sierra at 2600fps vs 3000fps is less than 3/4 of a minute at 200y. Most peoples cant shoot a 3/4 minute group, let alone have a rifle capable of doing so consistently.

Originally Posted by 6.5 CRD

Yeah your not wrong, Optical chronos can be a pain in the ass.

Calculating velocity from shot drops can work but its not a very accurate way of doing things imo. Alot of variables & not only that, need to shoot at some serious distance to try filter out the noise. 200y just isnt enough distance to give you an accurate idea of velocity. Difference in drop with the 150gn Sierra at 2600fps vs 3000fps is less than 3/4 of a minute at 200y. Most peoples cant shoot a 3/4 minute group, let alone have a rifle capable of doing so consistently.

yup fair call on the distance thing but probably still more accurate than an optical , I don't use distance for my initial velocity verification it as I have a magnetospeed, obviously drop should always be confirmed by actually shooting at range.