Velocity question

DGY

DGY

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good day all,

It seams like every load I develop I end up with lower velocity than what the book is getting, usually at least 100fps slower. I was wondering if it has anything to do with sitting the bullet as far as it is possible for the intended rifles?
What are your thoughts on that?
Could be my chronograph is reading differently than there’s, could be all my rifles have slow barrels, could also be…. Tell me !
Thanks
David
 
Could be your barrel length, new barrel, different barrel grooves, different temperature, powder lot variations, even different brand of brass can affect velocity. Could be a mixture of all the things I said.
 
It is my understanding that the "book" is telling you what primer they used, what brand of brass, how much powder, how deep they seated which particular bullet - and then tell you what they got for velocity in their barrel, with their chamber. As per Post #2, if you change any part of that recipe, is not really much reason why you would get the exact same results? In Nosler's case, they even report the grains weight of water they got within that case with that bullet seated to that depth.

I am not so sure that I go for the idea there is "faster" or "slower" barrel - but I do know there is cleaner, or more fouled; rougher or smoother; larger or smaller diameter; different groove and rifling widths and depths; longer or shorter barrels. In some cases, is even left hand twist versus right hand twist - not sure that affects velocity, but is another difference - one to the other. Chambers vary slightly in dimension, and is alleged that even the fit of the brass to that chamber can affect muzzle velocity.

For example - just the chamber length - SAAMI sets a Min and Max length - is apparently desirable for precision and target guys to want near or at perfect Minimum - even though a chamber up to .003" (or in some cases .009") longer will still be within SAAMI standards - which is what factories probably follow - they will (or should) pound out stuff between Min and Max lengths - not all the same. Apparently not what you pay for, to have a "precision" chamber done.

Perhaps leads to an over-reliance on gauges, measurements and numbers - I think you want your brass to fit snuggly - not .001" too long, or .001" too short. Is fussy to do - perhaps why SAAMI publishes a "range" of acceptable dimensions - until you measure and confirm, do not know what you have - but is most likely somewhere between Min and Max. Not likely we are set up to alter the chamber, so can get similar effect in how we re-size our brass - I think.
 
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BCBRAD said:
Find out what the jump to the lands distance is, may tell you something.
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Jump matters much less than the seating depth. A long throat allows for faster velocities.

But, the nose profile on the bullet can also influence things. Some nose profiles are so long that you cannot get near the lands and be short enough to feed from the mag. Such rounds will produce faster max pressure speeds if they sit further out of the case to touch the lands, even if you can only single feed rounds by hand.

Ideally you don't hit such a condition but it is quite common with 308 win for example.
 
I had my eyes opened a bit when I started loading for double barrel rifles.
For example, seemingly tiny insignificant differences in chamber and bore between the left and right barrels of one of my rifles has consistently shown 80-90 fps difference in velocity with the same ammunition. So it doesn't seem unreasonable that comparing different rifles from different manufacturers with different barrel lengths with different amounts of bore wear .... may make your results "vary"
 
Hitzy said:
Just means you can load up another 100fps if your barrel length is the same as the manuals.
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That would be true, if all barrels were created equal. They aren't and I'm not going on about barrel lengths.

It's very seldom that anyone can take two rifles with the same dimensions, same loads and get similar/identical velocities out of them.

Even with match grade barrels, chambers custom cut with the same reamer, you can expect anywhere from 50-100 fps difference in velocities with the same loads.

Increasing velocities, once you've reached the saturation point of your bore's capabilities can increase pressures disproportionately by as much as 10k+ psi to achieve that extra 100 fps.

I know where you're coming from though, just throwing in a word of caution
 
Book loads are often developed in a pressure barrel; a match grade barrel, tight, smooth and consistent from end to end then chambered to minimum spec then fitted to a universal receiver. It's pretty much a best case scenario; which you probably aren't going to find in run of the mill factory rifles.
Peak pressures are usually achieved in the first few inches of bullet travel when the majority of the powder is burnt, but a lot of speed is made after that by the still expanding gases. Better barrels better harness that pressure.

A pattern I've seen a lot is my custom barrelled rifles tend to match or exceed the book speeds, sometimes with charges less than the book maximums. Run of the mill factory rifles, you get what you get.
 
IvoB said:
Companies test fire their ammo using 24 inch barrels - if your rifle's barrel is shorter then that is why
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Not sure about that? I suspect they establish the load's pressure in a pressure test barrel, and then the velocity of that load in another barrel - so flipping through Speer #9 book - 308 Win is with Rem 700 and 22" barrel, 30-40 Krag with US Krag 1892 21" barrel, 30-06 is Rem 700 with 22" barrel, 300 H&H was Winchester Model 70 with 26" barrel, and so on. I think they usually tell you what they used, to get the results that they report??
 
If it's the Nosler reloading manual that you are using, well the same applies with me. Their velocities, for the most part, differ from my velocities which are lower than the manual.
 
DGY said:
good day all,

It seams like every load I develop I end up with lower velocity than what the book is getting, usually at least 100fps slower. I was wondering if it has anything to do with sitting the bullet as far as it is possible for the intended rifles?
What are your thoughts on that?
Could be my chronograph is reading differently than there’s, could be all my rifles have slow barrels, could also be…. Tell me !
Thanks
David
Click to expand...

Does 100 fps matter? Except for a very few situations, 100 fps will make no functional difference to your shooting or the bullet's performance.
 
I generally use a Nosler manual. I always knock off 100 fps compared to book and am generally pretty close. At least in my rifles.
 
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