Does seating depth and crimping affect velocity?

[elmerdeer]

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I have a few questions for you guys to answer,
1) Does seating the bullet .005 off the lands change velocity as opposed to seating .020 off the lands?
2)Does a light crimp make velocity slower as oppossed to no crimp?
3)I shot some reloads with my 7mm rem the other day, it was -15c or a 5f (cold) my loads were
RL22 66.5gr with 140 accubonds out of a 26" barrel,Sauer 202, my average speed with these loads was 2975fps.The Nosler book list that laod at 3240fps with a 24' tube, WHY THE BIG DIFFERENCE does the cold affect the velocity that much or is my barrel a bit slow? It is a new gun this was the 2nd time at the range with it.
Sorry for all the questions guys
Thanks
Elmer

 

[royinidaho]

Welcome aboard. Have a great stay!!!

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I have a few questions for you guys to answer,
1) Does seating the bullet .005 off the lands change velocity as opposed to seating .020 off the lands? <font color="blue">Yes, I believe you will find that the deeper the seat the lower the velocity. (here come the arguments) </font>
2)Does a light crimp make velocity slower as oppossed to no crimp? <font color="blue">I really don't know. Never crimped a rifle load. </font>
3)I shot some reloads with my 7mm rem the other day, it was -15c or a 5f (cold) my loads were
RL22 66.5gr with 140 accubonds out of a 26" barrel,Sauer 202, my average speed with these loads was 2975fps.The Nosler book list that laod at 3240fps with a 24' tube, WHY THE BIG DIFFERENCE does the cold affect the velocity that much or is my barrel a bit slow? It is a new gun this was the 2nd time at the range with it.<font color="blue">First I'd break it in properly then go to shooting. To respond to your question: That's a 265 FPS difference. If it were my 270 Win. with same weight bullet and powder type, 130 FPS would be due to the temperature change from summer to winter. That leaves 135 FPS for differences in chambers/barrels/bores/rifling and other variables. From my shooting of a 7mm Rem, you're running about par with factory ammo. </font>


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[Alan Griffith]

Not an argument on my part. Just reporting on a recent seating depth test I ran a few weeks ago. I started Nosler 180 BT's .005" into the lands; averaged 3040 fps at 31 deg F. I kept shooting 3-shot groups, loading deeper into the case by .005", until I hit my sweet spot, which was .015" off the lands. Vel was 3076 av at about 41 deg F. Powder was Rel 25 in my 30-06 Ackley Imp. Could it be the temp increase? Yes/maybe! Could it be seating the bullets deeper in the case? Yes/maybe! I don't consider a 35 fps increase a big difference. I had ES that was more than that!

 

[AJ Peacock]

Seating does make a difference in velocity, but I don't believe its a cut and dried relationship. This weekend, I shot some 7RM at different seating depths. The LONGER rounds actually shot faster (49fps on average 2994fps-&gt;3043fps) agreeing with Roy's experience. The longer of these rounds were VERY close to the lands (within .001"). (ES for the slower rounds was in the single digits, ES for the faster rounds was at 20+fps)

However, I have seen seating depth work the other way in other loads (reduced loads in .357mag rifle), where the deeper bullets actually made the light powder charge more efficient due to the smaller space. I've also seen a heavy crimp have the same effect. I've also seen deeper seating have higher velocities in a 7mm-08 with an exceptional amount of freebore (no way to get near the lands).

Seating depth can actually work both ways, depending on where you are with the load. As seating depth gets deeper, the usable volume inside the case goes down; this confines the powder charge in a smaller space (essentially making it act like 'more powder'). At the same time, you are getting farther from the lands, allowing the bullet to get a jump start down the barrel. Depending on the burn characteristics of the powder you are using, this smaller volume might make the pressure rise faster and might cause a higher velocity. On the other hand, with light neck-bullet tension, the bullet might actually start moving into the lands with only the primer pressure. If that happens, the powder is now burning in a larger space and might cause a lower velocity. But on the other other hand, the primer moving the bullet forward into the lands might cause the powder to burn at a higher pressure due to the bullets 'pause' in forward movement.

The pressure buildup and the influence pressure has on burning rates makes your question tough to answer in a general way. For the 7rm loads I tested this weekend, I think the longer loads shot faster because of being very near the lands and causing a steeper rise in pressure.

In general, I believe that if you are working a significant distance from the lands (.005" or more) and have enough neck tension to keep the bullet in place until the powder starts building pressure the shorter bullets will have a little higher velocity. Once the bullet/lands variable gets thrown into the mix, I believe the situation normally changes.

In essence, the answer will depend on the specific circumstances of your load/rifle.

As far as the temperature question: Quickload estimates your load at exactly 3000fps for a 26" barrel at 5deg F. It estimates your load at 3119fps for 70Deg F though. I have found reloading books to be somewhat optimistic at times.

Don

 

[britz]

Just throwing my 2 cents in. I have found that as long as you are not compressing loads, seating bullets deeper in the case reduced pressure signs on my 30-06.

 

[Alan Griffith]

For what it's worth, my loads were quite compressed.