COL vs PSI/FPS

[atmoshpere]

How much does your COL have an effect on your chamber pressure and load velocity assuming your bullet position relative to the lands remains the same?

As I understand it, if you could have your rifle throated to allow you to seat your bullets out as far as possible (assuming your mag allows a long COL) you would be maximizing your velocity potential for a given case.

Hypothetically, if you had two rifles chambered for the same round but one was throated for SAAMI COL and the other throated to allow say, a .20" longer COL, is there really that much difference in velocity potential? I'm assuming that the reloader is compensating for the change in case volume and adjusting his/her powder charge to maintain similar pressures with the two different rifles.

I have never loaded for more than one rifle in the same cartridge before, and was wondering if those of you who may have, have noticed velocity differences given the same barrel length, that may have been due to longer COL's in one rifle over another.

Thanks,

atm

 

[rick smith]

Won't have much effect unless you long throat so the bullet is out of the case before it touches the lands, WBY Mags.Rick.

 

[onesonek]

How much does your COL have an effect on your chamber pressure and load velocity assuming your bullet position relative to the lands remains the same?

Subject to the context of the rest of your post.....Anytime you alter case volume from recommended specs, you change the Expansion Ratio. You will see an effect, how much is hard to tell with any certainty

As I understand it, if you could have your rifle throated to allow you to seat your bullets out as far as possible (assuming your mag allows a long COL) you would be maximizing your velocity potential for a given case.

You run risk of feeding problems in doing this, generally speaking. But if you are contimplating this, leave some room in the magazine.

Hypothetically, if you had two rifles chambered for the same round but one was throated for SAAMI COL and the other throated to allow say, a .20" longer COL, is there really that much difference in velocity potential? I'm assuming that the reloader is compensating for the change in case volume and adjusting his/her powder charge to maintain similar pressures with the two different rifles.

Any two given rifles seldom shoot the same, so it's difficult to make any comparison. You take a the longer throated rifle that has a tight chamber, against a standard throat, with a somewhat roomy chamber,,,, you may not see any difference at all. In all likelyhood, one would expect the longer throat shooting the same bullet at the same pressure levels, one would see more velocity.
However, there are no absolutes in determining how much this would be. Each gun, is unto it's own.
Then too, keep in mind, say you do gain 50 fps,( I doubt you would see more , but possible ), you just added 12-13 yds. of effective range. Generally speaking, I figure roughly 25 yds. gain in PBR, for every 100fps gain. So, is it worth the expense, of the alteration.? Or in the case of custom build, worth risking the possible feeding problems?

I have never loaded for more than one rifle in the same cartridge before, and was wondering if those of you who may have, have noticed velocity differences given the same barrel length, that may have been due to longer COL's in one rifle over another.

I haven't either in the context you are inferring. I have however run the same cartridge with diffferent throat lenghts, in my single shots. But they throated for a secific bullet weight/lengths, for specific intended uses. And so, I haven't loaded them with the same bullet for a comparison.


Thanks,

atm

 

[atmoshpere]

Then too, keep in mind, say you do gain 50 fps,( I doubt you would see more , but possible ), you just added 12-13 yds. of effective range. Generally speaking, I figure roughly 25 yds. gain in PBR, for every 100fps gain. So, is it worth the expense, of the alteration.? Or in the case of custom build, worth risking the possible feeding problems?

This is kinda what I was thinking too. It seems like a topic that frequently comes up when people are considering a new rifle or building a custom one, and I always wondered how much difference it would make from a practical standpoint.

Thanks,

atm

 

[onesonek]

Well it certainly don't hurt anything to do so, if it fits the confines of the magazine without feeding issues. And I'll admit, I'm not overly familiar with most bolt action dimensions. However I would suspect, a short action has more limitation in this regard. On ther other hand, a custom build on a standard long action, with some intermediate rounds like the 57mm family, one would have some extra room, for long bullets seated out in order to get more usable case volume. But again, how much gain is hard to determine. I do so on mine because, most all my barrels are custom, so there is no added expense, and you don't have the length/ magazine issues with a single shot rig. It isn't so much because I want or need the extra few fps, it just because I can, without issues.

 

[Desert Fox]

Actually, the tightness of the chamber has more bearings on increase in velocity than the cartridge overall length. I'd experienced this first hand loading for my friends 300 Custom Weatherby with no free bore against my Factory 300 Weatherby Accumark with free bore. Both rifle have the same 26" barrel. For my rifle to matched the velocity of my friends rifle, I had to increased the load a grain and half more, and seat the bullet farther closer to the land.

 

[onesonek]

You sort of lost me there DF?
But then I could see your point to a point.
While the throat has a connection to the chamber, and cut at the same time as one in most instances, technically the throat is not part of the chamber. And then freebore is another concept altogether.
Freebore is nothing more than a longer throat, than what is considered standard. Usually expressed in calibers,,, such as a 2 caliber freebore, or in the case of a .308,, approximately .600" freebore throat measured from the ogive.
Initially freebore does two things basically, reduce pressure, and increase case capacity outside the case. The reduction in pressure, is why you need more powder to get the same velocity. But, that added powder, may have not brought your pressure up to what his is. It is not uncommon, and actually the reasoning behind freebore, to have to use more powder yet, to bring the pressure back to equal of his. So in the long run, you get more velocity at the same pressure.
Hence is why you gain case volume outside the case itself. And where Wby. gets most of it velocity increase over Mags of similar capacity.
Didn't mean this to seem a lesson, if you already knew this. Just sort of clearing the air for those that might not.

 

[atmoshpere]

Thanks for the info guys.

The cartridge I'm having a barrel made for is the 284 win, and from everything that i've read, with the 2.80" COL and 160 gr bullets you run out of case capacity before you reach max pressures with many powders. Most load data is generated using this 2.80 COL. My magazine allows a 2.98" COL. (I have run loaded dummy rounds through it at this length and it seemed to function fine and according to my mic, leaves me about .150" clearance at the top.) The rear wall of the magazine tapers slightly, making it shorter at the bottom, so I chose the 2.98" to make sure that a loaded mag wouldn't result in a sandwiched round on the bottom. So I should have about .18" more bullet on the outside of the case, which may make a bit of a difference on those loads that where compressed and still under max pressure by allowing me to fit a grain or two more powder inside the case. Pac-Nor is "throating" the barrel to allow my 160 AB round to "touch" the lands. These will be the longest bullets I'll shoot through this gun so from your explanation above onesonek (which I appreciate btw :grin: ) that means I'll have zero freebore with the 160 ABs at this COL right? So is freebore a variable that changes with seating depth? I always thought it was static... :?

Actually, the tightness of the chamber has more bearings on increase in velocity than the cartridge overall length.

Is this because a tighter chamber produces peak pressure faster due to a lower expansion ratio?

Thanks for the help, I learn something new on here everyday! Like RADD is contagious and can be contracted over the internet! :shock:

atm

 

[onesonek]

Thanks for the info guys.

The cartridge I'm having a barrel made for is the 284 win, and from everything that i've read, with the 2.80" COL and 160 gr bullets you run out of case capacity before you reach max pressures with many powders. Most load data is generated using this 2.80 COL. My magazine allows a 2.98" COL. (I have run loaded dummy rounds through it at this length and it seemed to function fine and according to my mic, leaves me about .150" clearance at the top.) The rear wall of the magazine tapers slightly, making it shorter at the bottom, so I chose the 2.98" to make sure that a loaded mag wouldn't result in a sandwiched round on the bottom. So I should have about .18" more bullet on the outside of the case, which may make a bit of a difference on those loads that where compressed and still under max pressure by allowing me to fit a grain or two more powder inside the case. Pac-Nor is "throating" the barrel to allow my 160 AB round to "touch" the lands. These will be the longest bullets I'll shoot through this gun so from your explanation above onesonek (which I appreciate btw :grin: ) that means I'll have zero freebore with the 160 ABs at this COL right?
correct


So is freebore a variable that changes with seating depth? I always thought it was static... :?
In the strict sense, no, it's not static. It is variable just as your barrel length is, as expansion ratio is caluated from the base of bullet. Although once you have a throat freebored 1-2 calibers, slight differences in seating depth, even with different weights, is really only relevant to that gun

Actually, the tightness of the chamber has more bearings on increase in velocity than the cartridge overall length.

Is this because a tighter chamber produces peak pressure faster due to a lower expansion ratio?
No, lower case capacity

Thanks for the help, I learn something new on here everyday! Like RADD is contagious and can be contracted over the internet! :shock:

atm