Home Forums Reloading Big Game Rifles Velocity difference due to seating depth, Barnes 120 TTSX

I ran some 120 TTSX from my T3 7mm-08 over the chrono today, with interesting results.

49.5 gr Big Game, WLRM primer. OAL of 2.835" averaged 2889 ft/sec, while OAL of 2.805" averaged 3002 ft/sec.

I tried some other loads and the longer OAL velocity was consistently lower than the shorter OAL. I did not shoot for accuracy, as I was shooting in my back yard.

OAL to the lands was 2.934", well past my mag box.

So does a longer jump increase velocity or am I doing something wrong?


P

It probably has more to do with how the case necks are grabbing the grooved bullets than it does with the difference in jump to the lands.

What you say intrigues me. Please expand on your thesis.


P

I'm just thinking out loud.

The difference in jump looks swamped by the size of the jump. But an .030" difference may change a groove from gripped to not gripped, or vice versa.

2.835" has the top of the neck resting on the shank a hair below the first groove. 2.805" has the top of the neck peeking into the groove.


P

Your load reaches Max pressure as the bullet is slightly down the barrel.
A combustion chamber is not just the brass but all the room from the base of the bullet to the bottom of the casing.
By seating deeper the bullet is not as far down the barrel making for an ever so slightly smaller chamber and a tiny bit of pressure/velocity increase.

4% seems significant.


P

Except that with bottleneck rifle cases and bullets off the lands, deeper seated bullets usually lower pressure. Yes lower, despite the slightly smaller start capacity.

I see that now..was thinking you had about a 20-30 fps diff.

From here I'd be looking at weighing those casings involved...or would question charge consistancy...or if a casing was in a warm chamber before firing. I routinely see 20-30 fps per. 030" bullet jump. Something else I would say is at play here.

Mathman...the only way you lower pressure by seating deeper is if you are moving away from a situation where you are so close to the lands that the bullet is struggling to get started down the barrel. Once you have a good enough run to the lands for the bullet to start easily seating deeper WILL not reduce pressure.

I have seen a graph of measured pressure vs seating depth where the pressure kept going down as the bullet was seated deeper. I'm talking reduced pressure until the bullet was a full quarter inch off the lands. Only going deeper than that started the pressure upward again. The OP has a long way to go before he's there.

Well.....the inventor of quickload and his 6-10 updated versions aren't gonna agree with that graph. Neither will my chrono. But we can agree to disagree.

When you shorten the OAL in Quickload, does the distance to lands remain at a fixed default or is it correspondingly increased?

Check out pages 47-48 of this document:

http://deepblue.lib.umich.edu/bitstream/handle/2027.42/3866/bac6873.0001.001.pdf?sequence=5

All of my experiments with deeper bullet seating in rifles back up what mathman states, and have for 20+ years. Longer throats do the same thing: reduce pressure and velocity with the same load.

I don't trust QL pressure predictions much, due to several experiences with actual pressures vs. QL predictions.

I'm also guessing that extra "bullet pull" due to more bullet-shank contact with the neck is the reason for the extra velocity with deeper seating.

Quickload isin't perfect but its good enough that several major bullet and powder companies use it to run scenarios and send them out to their customers via email. I know this from requesting info from several after running my quickload and was shocked to see exact matches when I go their downloads.

I just pulled out my copy of Stan Watsons "handloaders odyssey" book. I really enjoyed his through testing of the good old 3006.
I checked his graphs and info when he used seating depth as a variable. The highest pressure as can be expected were when the bullet was jammed, just touching and .019" off the lands. Pressue went down when .039" off the lands....went up slightly when .059" off the lands...then dropped slightly at .164" off the lands. The last three pressures were within 900 psi...virtually insignificant. Except for being jammed velocity varied by only 20 fps...again virtually insignificant.
I understand this is a test of one.
I will also say that quickload is very hyper about pressure rises when bullets are seated deeper....but has always factored it to be a pressure increaser.

Spent a little time looking at resources about this.
Pretty funny...my hornady manual says seating deeper gives less pressure because the bullet moves further faster w/o the rifling to slow it down.
Bryan Litz...ballastician for Berger says the opposite...pretty much a mirror of my "increased" chamber theory....
http://www.bergerbullets.com/effect...and-cartridge-base-to-ogive-cbto-part-1/
Apparently we're not even gonna get two major manufacturers to agree lol!

That's interesting.

As for "major manufacturers," yes, some will suggest QL data for customers, but ONLY after they've already tried the powder in question at least a little in that cartridge, and even then they are very conservative.

One major bullet manufacturer used QL several years ago to put together a bunch of ammo to send to a major pressure lab for testing. About 1/3 was spot-on, another 1/3 was way under-pressure, and about 1/3 was way over-pressure.

I've used QL a lot for a number of years, but it's a simulation, not reality. I especially don't trust it concerning predicted pressures, because have seen them way off too often.

I just read Bryan's write-up, and there are a lot of unanswered questions, such as the bullet relationship to the lands.

I've done soe experimenting with seating depth, both on my own with s chronograph, and in various ballistic labs with both strain-gauge and piezo equipment, and have yet to see velocity or pressure increase when bullets were seated deeper in the same rifle--though haven't seated them more than about .25 off the lands, when the throat allowed the bullet could reach the lands in the first place.

The speed of the bullet was addressed, but I am curious as to the directional difference created by the altered seating depths.

Mule Deer : ever do any experimenting with 257 Weatherby Mag. on seating depth? i do know Weatherby designed his cartridges to be jump started so i never messed with seating depth , but kinda always wondered on seating depth too ?