Longer barrel = increased velocity ? Discuss

scrun63

Well-Known Member
Until yesterday I was sure that the longer the barrel the greater the velocity for a given powder/bullet combination. However talking to an RFD yesterday he said this was not the case (specifically about a 17 HMR). His reasoning being that the modern powders have all burnt within a short distance from the chamber. Is he correct ?

Answers gratefully recieved

Scrun
 
Hi Scrun, Not in my opinion.
I have two rifles in 6.5x55, a Sako of 23 inch barrel length and the stutzen of 19 inches. With Hodgdens, IMR and Vihtavouri powders, through the chronograph the stutzen is allways between 80 and 100FPS slower than the Sako.
 
I would make only two points:

1. What your RFD said may well be true relating to rimfire rounds, but I would never look to extrapolate that to any general rule for CF rounds.

2. There is more bullsh!t talked per square yard in gunshops than anywhere else in the known world. The crap I have listened to while biting my tongue in such places woulg begger belief.

As a general rule, for CF rifles, longer barrels mean more velocity, but only up to a point, maybe 30" or so. However, some rounds suffer from velocity drop off much more than others. The bigger the bore size relative to case size, the less impact shortening the tube will have. So a 308 will suffer rather less velocity loss than say a 270win.
 
it is not as relevant on rimfire as it is with centre fire (even smaller calibre ones)
the volume of propellant in rimfire is very fast burning (lil'gun-esque if not exactly the same) and is probably as your RFD describes.
its why many people shoot 22lr and even HMR with 12-16" barrels without issue or huge reduction in velocity

it is rumoured to be 50-100fps loss per inch of barrel reduction (calibre and load dependant)
 
.22 L/R burns completly in something like the first 16" so anything longer can actually, in theory anyway, start to slow down. When my old and slightly battered BSA Supersport Five was cut and threaded it was cut to 16 1/2" and then threaded so in theory should produce the best velocity out of the ammunition.

Centre-fires are different beasts and a lot depends upon the burning rate of the powder. My BSA CF2 Stutzen with it's 20" barrel produces an impressive fire ball at the muzzle with most factory and recommended 270 Win handloads so I switched to using a faster powder, H-380 in this case, which has reduced this effect.

The .17 MHR is based upon the .22 L/R so it probably follows the same rationale and burns in abut the same time frame. I am no ballistics expert but rifle powders have to be of a progressive burn or pressure spikes would be too high for safety. With 60grains of powder igniting almost instantly I doubt many rifles would survive for long before suffering damage. Bear in mind soem of the lager cases magnums use much more than 60 grains of powder.
 
Thanks for the info. As always things appear not to be as black or white as they first appear

Cheers

Scrun
 
Cant remember where but have read a simular theory relating to the .17hmr that the optimum barrel lentgh is 16" after which all powder has been burnt so in theory friction will slow it down,probably explains why all that crud falls out the end of my 18" barrel :lol:
 
it is not as relevant on rimfire as it is with centre fire (even smaller calibre ones)
the volume of propellant in rimfire is very fast burning (lil'gun-esque if not exactly the same) and is probably as your RFD describes.
its why many people shoot 22lr and even HMR with 12-16" barrels without issue or huge reduction in velocity

it is rumoured to be 50-100fps loss per inch of barrel reduction (calibre and load dependant)

Most of the reading I have done on this subject would suggest that 50-100fps per inch is actually a bit much, with some reference being made to only 30fps per inch. It changes though as the barrel is increased or decreased. It is not quite as simple as saying 30fps per inch across the whole barrel length if that makes sense.

I also read that with .17HMR in particular, anything beyond 16'' offers no real benefit at all.

If you look into it, as I did a while back, certain calibres because of their bore size lend themselves to short barrels while still retaining good velocity because of the powders they can burn. One that springs to mind was the 7mm-08AI. I don't know if I can find the article but it said that this was one of the few that could still break the 3000fps mark from a 20'' barrel while using hunting bullets around the 140grn-150grn weight.

25-06 was one I recall being referred to as benefiting from a longer barrel (at least 24'') because of the slower burn rate powder that worked well in it. I practical stalking terms does any of it make any real difference? I personally would suggest not as an extra 100fps or so may only make a difference in drop of .1 or .2 inches at 200yds from a 100yd zero.
 
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There are a few other things to consider while we are at it.

I cut my 260rem from 22.5" to 20.5" and it cost me 45-50fps total. That is measured on a chronograph before and after. 25 FPS is not a bad rule of thumb for the 308 cased rounds, though the 243 may fair a little worse.

The 30/06 based rounds will be worse than this, I would have thought a 25/06 or a 270 would loose near the 50 FPS per inch mark. I would not want to own a 270 with less than a 22" barrel. I do have a short barreled 30/06 which is not too bad and gets within 100 fps of book velocity with a 20.5" barrel.

Also, the relationship between barrel length and velocity is not linear. Cutting say a 308 from 26" to 22" will have a lot less effect than cutting it from 22" to 18", ie the shorter you go, the more velocity you will loose per inch of barrel.
 
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A super long barrel with reduce the potential velocity of the bullet. The frictional forces acting on the bullet will remain constant while the maximum pressure achieved reduces as the volume of space for the gases to fill increases during bullet travel to muzzle.


my hypothisis anyway
 
Until yesterday I was sure that the longer the barrel the greater the velocity for a given powder/bullet combination. However talking to an RFD yesterday he said this was not the case (specifically about a 17 HMR). His reasoning being that the modern powders have all burnt within a short distance from the chamber. Is he correct ?

Answers gratefully recieved

Scrun

with a light load of powder.
4-20 grs thats possibly correct
I reduced my hornets barrel to 14" and was using 11grs. I lost about 40fps or so.
But on larger shells and slower powder longer is better.
Pointless having a true magnum with a 22" barrel or less.
When i re-barrel my 22250 it will be an AI and i will go 27"
 
The RFD is correct about 17hmr.
All this is pretty achardemic for deer stalking as far as I can see, so long as you stay above 20". RPA know a thing or two about rifles, thier latest has 16" barrell !
 
This is an interesting question. I’d always understood the rule of thumb to be:
Up to 3000 fps velocity, increase/reduce by 20fps per inch, over 3000 fps increase/reduce by 30fps per inch from stated load velocities in a manual.
However, I use a 20.5" barrel and expected a reduction of 30 fps per inch from the data given, their test barrel being 24" long.
What I actually chronographed was in fact almost exactly as stated for a 24" barrel, this, with an extreme spread of 22 fps. in my readings.
I was surprised and pleased.
So, I have to assume the data given in the older manuals isn’t necessarily applicable to the newer extreme propellants available now.
Cheers, :old:........I just don't know!:confused:
 
I really dont know enough about this to be sure but if the powder burns say in the first 8 inces of the barrel there must still be preasure beind disapated up the barrel. the longer the barrel the more energy put into the bullet head? Also If the powder is burnt in the first few inches of the barrel why is there muzzle flash of what I can only think is burning powder out the end of the barrel.

Mark
 
OK Alan, - Chuck Hawks agrees/confirms velocity data published by some respected manuals. But this doesn’t explain why chrono results in my case contradict/differ from the various published data.
I have to presume the chrono I use is working fine & the readings are reliable.
I have no reason to believe otherwise. Of course, I cannot say my tests were under ‘Lab’ conditions, nevertheless, it is live data, taken from my loads. My rifle is a 20.5" barrel K95, from which I should have expected around -3.5x 30fps = 105fps less velocity than published using a 24" test barrel.
In fact, I got on average 108fps more.
The truth is however, whatever may be the explanation for this, the deer don’t know and will care less…..but, it is curious.
Cheers, :old:
 
Altitude, temp, and your bore condition etc can all effect muzzle velocity.
you would need to get your own results from a barrel before and after cut under similar conditions.
NOT compare to a book velocity and test barrel.
 
OK Alan, - Chuck Hawks agrees/confirms velocity data published by some respected manuals. But this doesn’t explain why chrono results in my case contradict/differ from the various published data.
I have to presume the chrono I use is working fine & the readings are reliable.
I have no reason to believe otherwise. Of course, I cannot say my tests were under ‘Lab’ conditions, nevertheless, it is live data, taken from my loads. My rifle is a 20.5" barrel K95, from which I should have expected around -3.5x 30fps = 105fps less velocity than published using a 24" test barrel.
In fact, I got on average 108fps more.
The truth is however, whatever may be the explanation for this, the deer don’t know and will care less…..but, it is curious.
Cheers, :old:

The problem with chronographs is that they all have to be set identically to give valid comparative readings. Even small differences in muzzle to first screen distance can influence the figures. When I was involved in loading in a factory environment the test gun and screens were permanently fixed in place. Even then direct comparison with other equipment was not accurate due to sometimes small differences in set up. A chrono is a good tool but don't believe that the figure it gives is actually what the round is delivering. They are more useful for comparison between different loads tested on the same equipment.

Alan
 
I can certainly accept in order to deliver exact comparisons, all equipment & conditions involved necessarily must be identical for each test.
However, if the chrono data shows details for my particular load, then at the time & conditions tested, that’s what the load/rifle was delivering.
Naturally temperature/altitude changes in the field will be different, changing matters entirely – but at least I have an indication of what the load is capable of.
Cheers, :old:
 
Another thing t remember is that different rifling forms behave differently, some yield better velocity than others, then as already pointed out errosion will have an effect if compared to a new un worn barrel.
 
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