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bullet seating depth & high pressures

 
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  #1  
Old 12-04-2009, 07:44 PM
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bullet seating depth & high pressures

how much does bullet seating depth affect the pressure of a load?for instance can the bullet be seated too deep into the case and cause high pressure?
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Old 12-04-2009, 08:25 PM
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Re: bullet seating depth & high pressures

Typically, the deeper a bullet is seated the less pressure and velocity will be generated. Reference the following chart from Barnes

September 2007 Barnes Bullet-n | Barnes Bullets



where the first loading on the left in each caliber is closest to the lands and going to the right they are seated deeper by .050". The pressures are generally lower as the distance from the lands increases.

Also this graph which I don't know the origin of



Hornady also documents this with some explanation of

Internal Ballistics - Hornady Manufacturing, Inc

"To illustrate the effects of variations in bullet travel before the bullet enters the rifling, we'll compare a standard load with adjustments made only in the bullet's seating depth.

In a "normal" load with the bullet seated to allow about one 32nd of an inch gap (A) between the bullet and the initial contact with the rifling, pressure builds very smoothly and steadily even as the bullet takes the rifling. Pressure remains safe throughout the powder burning period (B), and the velocity obtained - 3500 fps - is "normal" for this load in this rifle.

Seating the bullet deeper to allow more travel before it takes the rifling, as in these next two illustrations, permits the bullet to get a good running start (C). Powder gases quickly have more room in which to expand without resistance, and their pressure thus never reaches the "normal" level. Nor does the velocity; with the same powder charge it only comes to 3400 fps (D).

When the bullet is seated to touch the rifling, as in the accompanying illustrations, it does not move when the pressure is low (E); and not having a good run at the rifling as did the other bullets, it takes greatly increased pressure to force it into the rifling. As the rapidly expanding gases now find less room than they should have at this time in their burning, the pressure rise under these conditions is both rapid and excessive (F). Velocity is high at 3650 fps - but at the expense of rather dangerous pressure. Many rifles deliver their best groups when bullets are seated just touching the rifling. Seating bullets thus can be done quite safely if the reloader will reduce his charge by a few grains. The lighter load will still produce the "normal" velocity without excessive pressure. "


All this being said, pressures will also depend largely upon the size of the combustion chamber in your rifle. IOW, if you have a long throat that exta volume at the long leade will be a part of the equation that dictates pressures so you have to use the prologue "All other things being equal....................."
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Old 12-04-2009, 08:50 PM
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Re: bullet seating depth & high pressures

thanx woods
the reason for my question is i had to back my loads down by quite alot because of pressure signs and i wasnt happy with my velocities afterwards. so i loaded a couple of rounds that i knew was high pressure so my smith could assist in troubleshooting he thought maybe my chamber is on the low side of spec and didnt think he would need to fire the rounds so when i got home i proceeded to disassemble the rounds with my rcbs bullet puller (hammer) and i noticed how much more difficult these loads were to disassemble than other calibers i have disassembled and i was curious because i did seat these pretty deep in the neck of the case

thanx again
i really appreciate the advice
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Old 12-04-2009, 09:54 PM
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Re: bullet seating depth & high pressures

Then I guess the next questions would be:

How far off the lands are you loading?
Do you have a magazine restriction, IOW do you load to fit the mag or load to fit the lands?
What caliber and what powder are you using, IOW can you gain velocity back by going to a slower powder?
What is your bullet grip, sounds like it is strong, maybe more than necessary?
What is the length of your barrel?
What bullet you shooting?
What primer you using?

I will say this, a small chamber will give higher velocities. I have a 338RUM rebarrel that was made with a short throat and small chamber and if hits max load (and higher velocities) a lot quicker than the manuals would indicate. A larger than spec chamber generates less pressure and less velocity than the other way around.
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Old 12-04-2009, 10:37 PM
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Re: bullet seating depth & high pressures

this particular rifle is my 220 swift that i just had rebarreled with a 22" lilja stainless.i never checked how far from the lands the bullet is seated i just started these loads at the same cartridge length i had with the old barrel and the accuracy is superb it shoots the 50 gr sierra blitzking 3 shot groups consistantly at .340 @ 100 yds with an occasional .250 three shot group but i had to drop down to 3200 fps to get away from the pressures (i.e. stuck bolt).this load is using imr 4064 powder and fed 210 primers. i am very happy with the accuracy just not the velocity, 3200 fps is 223 velocity not swift velocity.i do realise i will lose some velocity with the shorter barrel but i wouldnt think i lose this much.
and the reason i went to a 22" barrel is i wanted a faster handling rifle for coyotes
i also shoot a 300 RUM but i dont have any issues with it just YET!
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Old 12-04-2009, 10:51 PM
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Re: bullet seating depth & high pressures

"for instance can the bullet be seated too deep into the case and cause high pressure?"

You (finally!) assure us you're shooting rifle. Okay...

As a general rule, seating deeper for rifles actually reduces peak pressure a tad, so long as it's rational. But all bets are off if you push the bullets though the neck! (I only add that part because there's often someone who wants to take things to extremes to voice an objection! )

Handguns, with small case volumes, fast powders and moderately heavy bullets, seating significatly deeper does raise peak pressures, sometimes markedly so.
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  #7  
Old 12-05-2009, 12:02 AM
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Re: bullet seating depth & high pressures

That is terrible velocity for a 220 Swift.

How many grains of IMR4064?
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