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  #1  
Old 05-28-2013, 05:21 PM
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Effect of number/type of grooves on velocity


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Hey guys/gals,
My question relates to how much influence does the number and type of groove cut/formed in a barrel effect velocity?
The reason I ask is that I have 3 rifles in 300WinMag and they all shoot the same loads differently, I fully understand the initial reasons for this, such as slight variations in groove/bore dimensions etc. I have also pressure tested each rifle with the same loads and the difference in pressure is only slight, less than 5000psi to be exact.
One rifle has skinny 6 groove rifling, one has wide ballard type 6 groove rifling and one has equidistant 4 groove rifling.
Does anyone have a theory on this?
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Old 05-28-2013, 05:32 PM
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Are all the twist rates the same in each rifle?
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Old 05-28-2013, 06:21 PM
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Well, you missed the boat partially. The reason for shooting differently isn't because of groove/bore differences alone.
Certainly that can be a part of it, and by your pressure trace results; proves there isn't much difference between grooves and pressures.
What you have to consider is the whole systems, and not the parts. Those rifles all have the the potential to have differences in:
Chamber dimensions, free bore, bore/groove dimensions, how sharp the rifling ramps to full height, barrel length, barrel harmonics, rifling.

The most likely cause for the pressure differences, are in the chamber and freebore differences.
The most likely cause for the differences in where they shoot, is barrel harmonics.
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  #4  
Old 05-29-2013, 04:35 AM
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Guttshooter,
Yes, all barrels are running a twist of 1-10".
The biggest difference is between my Kimber 8400 and my Rem sendero/Win Mod70, all having 26" barrels. The Kimber regularly gets 100-150fps more velocity with the same loads and similar pressures as does the Sendero/Winchester. The Kimber has equidistant 4 groove rifling, the Sendero has skinny lands and wide grooves with 6 grooves. My Win Model 70 has equidistant 6 grooves and shoots the same loads within 25fps of my Sendero.
BTW, only the Kimber has a chrome moly barrel, the others are both stainless.
Cheers.
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Old 05-29-2013, 12:30 PM
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Does the Kimber have standard 90 degree lands or are they canted, like a 5R type or 5C type. The 5R or Broughton 5C type barrels generally will produce anywhere from 75fps-200fps faster muzzle velocities than standard 90 degree lands and grooves do. I would think the 4 groove having 2 less lands to dig into the bullet would change the velocity as well. The point of impact could be different as stated above by velocity, barrel harmonics, bedding, ( the Kimber is both glass and pillar bedded ) and not sure what kind of stocks you have on the other two. If the Remington has a Walnut stock then there is no bedding at all, the Winchester synthetics if it's newer all have a Bell and Carlson full bedding block in them, if it's walnut then I don't think they have any bedding either.
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Old 05-30-2013, 12:35 PM
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I would think that the greater number of grooves and lands would give higher pressures than lesser ones.
You stated the difference of 5,000 PSI.
That's not a slight difference.
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Old 05-31-2013, 07:27 PM
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Way back in the 50s a buddy had a Marlin 22 with seems like 16 groves.. I dont think it shot any different than my Mossberg.
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Old 06-04-2013, 04:44 PM
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Quote:
Originally Posted by fred243 View Post
Does the Kimber have standard 90 degree lands or are they canted, like a 5R type or 5C type. The 5R or Broughton 5C type barrels generally will produce anywhere from 75fps-200fps faster muzzle velocities than standard 90 degree lands and grooves do. I would think the 4 groove having 2 less lands to dig into the bullet would change the velocity as well. The point of impact could be different as stated above by velocity, barrel harmonics, bedding, ( the Kimber is both glass and pillar bedded ) and not sure what kind of stocks you have on the other two. If the Remington has a Walnut stock then there is no bedding at all, the Winchester synthetics if it's newer all have a Bell and Carlson full bedding block in them, if it's walnut then I don't think they have any bedding either.
I cannot answer honestly which type of rifling the Kimber has.
Apart from the Kimber, which is untouched from the factory, both the Sendero and Win are highly tuned heavy barrel rifles with HS stocks proffessionally bedded etc.
I never mentioned point of impact differences, and that's not what I'm concerned with, only the massive velocity differences with similar pressures.
Your answer goes along with my way of thinking.

Easternhunter,
A 5000psi difference in a load almost makes no difference to a velocity spread, I have tracked many loads that spreads higher than this from primer substitutions alone and the velocity hardly changed. In fact I've seen loads swing more than this with poor ignition problems.

BTW, all chambers are running the standard 3 degree tapered throat and the throat lengths have been adjusted to be the same as the Kimber which had the longest throat of the 3 rifles.
Cheers.
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Old 06-05-2013, 11:58 AM
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I totally agree,
There's a thing called diminishing returns.
Up to a point, more propellent rarely gives proportional velocity.
But, 5,000 psi is a significant increase of pressure in any rifle.
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  #10  
Old 06-05-2013, 05:03 PM
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Quote:
Originally Posted by MagnumManiac View Post
I cannot answer honestly which type of rifling the Kimber has.
Apart from the Kimber, which is untouched from the factory, both the Sendero and Win are highly tuned heavy barrel rifles with HS stocks proffessionally bedded etc.
I never mentioned point of impact differences, and that's not what I'm concerned with, only the massive velocity differences with similar pressures.
Your answer goes along with my way of thinking.

Easternhunter,
A 5000psi difference in a load almost makes no difference to a velocity spread, I have tracked many loads that spreads higher than this from primer substitutions alone and the velocity hardly changed. In fact I've seen loads swing more than this with poor ignition problems.

BTW, all chambers are running the standard 3 degree tapered throat and the throat lengths have been adjusted to be the same as the Kimber which had the longest throat of the 3 rifles.
Cheers.


You had to of worked those loads up from some know data and 100/150fps difference is big deal comparing start load/velocity to max load/velocity. I load mostly from Nosler and Hodgdon manual and Nosler max load for 175/180 gr bullet for the 300mag velocity spread is 180fps.
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Old 06-06-2013, 02:59 AM
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Originally Posted by old roper View Post
You had to of worked those loads up from some know data and 100/150fps difference is big deal comparing start load/velocity to max load/velocity. I load mostly from Nosler and Hodgdon manual and Nosler max load for 175/180 gr bullet for the 300mag velocity spread is 180fps.
I really do not know what point you are trying to make?
The velocity difference is with max loads, not comparing start loads to max loads.

All of my data has been worked up by myself, I started using RE25 well before any useful data was published. After extensive testing RE25 gives the highest velocity with the least pressure than every other powder I tested, in relation to bore capacity it is quite efficient.

Cheers.
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Old 06-15-2013, 06:13 PM
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Magnummanic,
Just wondering how you did pressure testing on your rifles?

A few things you might sort out that can maybe explain variations in velocity.


Did you measure bore diameters with a air gage or ???????

Obviously all three chambers were not cut with same reamer and differences in chambers will give you lots of variation.

Did you measure the throat dimensions to determine how far bullets move before contacting rifling?

Casting throats and taking measurements might prove very educational.

Have you looked at bores with a good bore scope (Olympus Series 5 for instance) to determine surface finish.

Also have you borescoped to see just how much bullet contact there is in the grooves. I believe you will find a considerable difference in the amount of bullet contacts in the grooves which you can determine by looking at the contact widths of wear the bullets leave. From experience I can tell you there are lots of different wear patterns to be observed from heavy contact in grooves to little or no contact in grooves. Obviously no contact in grooves is going to let lots of gas flow past the bullet before it gets to the muzzle.

Did you take the pressure measurements using the same cartridge case or different cases?

Did the primers come from same lot number? I ran a series of tests about ten years back where I loaded the same cartridges with the same lot of propellant from same box of bullets and ran 33 round strings fired at the same ambient temp, one round every 60 seconds but used primers from three different manufacturers and the variation was enough to drive one to forget it and go get a beer. In short I fired 33 rounds recording all velocities, cleaned barrel, reloaded all the ammo changing only primers and back to the bench.

Ambient temp is critical as I found out running proof series on 40MM air defense guns at Aberdeen Proving Ground. There is no factory loaded "proof" rounds for 40 MM so AP rounds were pulled down and the propellent was added giving about five different charges and we went out and shot them quickly and brought the pressure vessels in and they were measured to determine the window we were looking for (70,000 thru 75,000 lbs) and the optimum charge weight was loaded into about ten more rounds and we were off to the proof arena and we shot them as fast as the barrels could be changed out and then went out and found the pressure gages and brought the pressure cylinders back for gaging as quick as we could.

Just about every day we had to run another pressure series after lunch as the ambient temp had risen.

On winter days when it was overcast the temp stayed pretty much even but if the sun came out we found the pressures starting climbing and say we started with 71,000 lb loads, if the temp came up say ten degrees before we got through we could exceed the 75,000 window and that required a trip to inspection and the barrels were mag particle inspected to determine if we had cracked one. I think in 75 tubes we overpressured on about three of them due to temp rise between 1000 and 3000 lbs over max pressure but none were cracked.

For instance the average velocities changed, the SD changed and the groups changed and that was just one rifle.
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