Calculating backthrust?

[securitysix]

Ok, I understand that the weakness of the T/C Contender is backthrust, and this is what keeps it from being able to chamber certain cartridges.  I've also done some googling and have found a couple of different places that say backthrust = chamber pressure * internal base area of case.  Some backthrust is eaten by the cartridge gripping the chamber walls, and I've seen no formula to calculate and/or correct for that.  Being bored, I calculated some backthrust figures:

.223 Remington should generate roughly 4761 lbs of backthrust (55,000 PSI SAAMI Max pressure)
.44 Magnum should generate roughly 5905 lbs of backthrust (36,000 PSI SAAMI Max pressure)
.30-30 Winchester should generate about 5846 lbs of backthrust (42,000 SAAMI Max pressure)

These are regularly chambered in Contenders.

For comparison, the .458 Win. Mag. puts out 6929 lbs. of backthrust (53,000 PSI SAAMI Max pressure).  I've never seen or heard of a Contender chambered for it, and I don't think I'd fire one if I came across it.

SAAMI max pressure for the 8mm Mauser is 35,000 PSI, meaning the 8mm Mauser will put out 4598 lbs of thrust if it's loaded by Remington or Winchester.  CIP sets the max pressure for the 8mm Mauser at 57,000 PSI, which yields 7488 lbs of backthrust.  Obviously, firing CIP specced (Mil-Surp or European commercial) 8mm ammo wouldn't be safe in a Contender, which is why you don't see Contender barrels chambered for 8mm Mauser.

But the question this brings up (and if you're still reading far enough to get to the question in the post) is this:  What is the maximum safely allowable backthrust that a Contender (or G2) frame can take?  Same question with regards to the Encore, as well.

[Scibaer]

six,
 i been working on this same issue myself for a wee bit now. see the thread here, http://www.gboreloaded.com/forums/index.php/topic,182799.0.html
if you read thru all that, you will find that the number you are looking for is a bit elusive.
calculating the backthrust is a bit tricky, you need numbers for weight of bullet, powder, speed of powder burn rate, speed of bullet, time in barrel, max SAAMI pressure for that chambering, area of brass in case, brass head area coverage of frame, area of brass in chamber of barrel , barrel lenght and atmospheric pressure to get a actual correct backthrust number.
 but when you get it, then what ? the next time you load the contender, those numbers will be different, slightly so, but still different.
so, in short i gave in. i ended up conceding and saying that 6000 psi was max for the contender and G2 frame, with 40,000 cup being max for the case pressure. i use my contender for hunting, its a 44mag. if i even come close to those numbers, its all anyone wants to handle for more then a few shots.
  if you are talking about a rifle round,that is not factory chambered for the TC ? i'd say let the pro's at TC do all the work and testing first. that IMHO and .02 worth, lol ..
glenn

[securitysix]

six,
 i been working on this same issue myself for a wee bit now. see the thread here, http://www.gboreloaded.com/forums/index.php/topic,182799.0.html
if you read thru all that, you will find that the number you are looking for is a bit elusive.
calculating the backthrust is a bit tricky, you need numbers for weight of bullet, powder, speed of powder burn rate, speed of bullet, time in barrel, max SAAMI pressure for that chambering, area of brass in case, brass head area coverage of frame, area of brass in chamber of barrel , barrel lenght and atmospheric pressure to get a actual correct backthrust number.
 but when you get it, then what ? the next time you load the contender, those numbers will be different, slightly so, but still different.
so, in short i gave in. i ended up conceding and saying that 6000 psi was max for the contender and G2 frame, with 40,000 cup being max for the case pressure. i use my contender for hunting, its a 44mag. if i even come close to those numbers, its all anyone wants to handle for more then a few shots.
  if you are talking about a rifle round,that is not factory chambered for the TC ? i'd say let the pro's at TC do all the work and testing first. that IMHO and .02 worth, lol ..
glenn

Oh, you're right, about all the other things that go into the actual calculation.  But if a guy is wanting to do some sort of wildcat round completely on his own, it'd be nice to have a place to start.  Sure, you have no exact pressure figures to work with when wildcatting, but if you know what pressures the parent cartridge runs at, it can give you an idea of the area you're working in.  I guess I could email T/C.  Wonder if they'd be willing to actually answer such a question...

[LONGTOM]

Sure they will answer it in two ways.

NO!
&
We will not discuss reloads or wildcats.
These were the answers I got sometime ago when I first inquired about the possibility of a WSM cartridge in an Encore.


LONGTOM

[NebrHunter]

What I was told about this question was that it has to do with the back thrust
vs the dia. of the shell case. A 44 mag. has a bigger area than a 308 win, and so on.
Get what I mean ?     mc   

[quickdtoo]

What I was told about this question was that it has to do with the back thrust
vs the dia. of the shell case. A 44 mag. has a bigger area than a 308 win, and so on.
Get what I mean ?     mc   

The 44mag has a .457" case head, the 308Win is .470", rim diameter is not a factor in computing back thrust, it's the internal case head dimension. http://stevespages.com/page8d.htm

Tim

http://www.riflebarrels.com/articles/custom_actions/bolt_lug_strength.htm

Bolt Thrust

Bolt thrust is easy to calculate. Only two inputs are required. They are peak chamber pressure in PSI and as mentioned, the inside area of the case head that the gas pressure can work on. The formula then is:

THRUST=AREA*CPSI Where:

AREA=3.1416*(HS/2)^2

HS=the diameter of the inside of the case head.

I sectioned some cases and measured the inside diameters and found that they were as follows:

CARTRIDGE CASE    INSIDE DIAMETER (HS)
222                            .300"
PPC                            .370"
308                            .385"
MAGNUM                        .420"
378 WBY MAG              .500"
50 BMG                        .680"

[Graybeard]

I keep seeing folks commenting that thrust is based on the inside diameter of case not the external diameter and generally always quoting the same source for that info.

I'm not saying that's wrong but then I'm not saying I believe it's right either. I must say tho that it doesn't make good logical sense to me. The head or base of a brass case is solid enough that whatever pressure is applied inside the case where I do agree it is acting directly is also transferred to the outside aka base of the case. Now to me logic says that pressure is thus acting on the diameter of the base whether rimmed or rimless. Dunno to be honest but this whole mess seems a round about way for folks to try to justify using rounds in the Contender and to some extent the NEFs that the factory and most all after market barrel makes have said are not suitable.

I'd sure feel better if the data was coming from a more reliable source than Steve.

[quickdtoo]

The reference to Steve's pages is for cartridge dimensions only, the bolt thrust reference is from Lilja.

Here's an article on breech thrust at Realguns with some calculated data for common levergun cartridges.

Tim

http://www.realguns.com/Commentary/comar52.htm

Some things I don't know about bolt thrust....

Lately, bolt thrust seems to come up whenever there is discussion about the pressure handling characteristics of a particular firearm and cartridge combinations, i.e. "You can rebarrel your Chipmunk and make the change from .22 Hornet to .577 N.E., but beware of the increased bolt thrust". Why be concerned about bolt thrust at all ? Forget those giant 2 or 3 or 9 or more bolt action lugs, or even that rotating lock up of a Savage or Browning lever gun. The Marlin has basically three small pieces of metal holding that bolt in place behind the cartridge, maybe 3/8" thick at the widest point and thinning dramatically behind the forward "lug" on the lever. I guess I can include the pivot screw that holds the lever in position as part of the overall support equation.

Bolt thrust is one of those firearm strength indicators, or probably more correctly stated, forces working against strength indicators, that is of little use without knowing the strength of resistance. Without a baseline to define acceptable or excessive thrust, there is little significance in the indicator ?

Most recently I've seen the formula for bolt thrust expressed as being equal to the area of the smallest interior surface of the case web perpendicular to the bore, multiplied by the chamber pressure, and expressed as p.s.i1. This is similar to determining the compression strength of a column by calculating at the smallest diameter, but I don't believe this can be expressed as p.s.i because bolt thrust is suppose to be an aggregate pressure exerted over an area, the surface area at the case web perpendicular to the bore. As a relative indicator it works pretty well. I arrived at a radius by dividing the interior case diameter by two, I squared the result and multiplied by 3.14 to get the surface area. I multiplied the area by the case pressure and expressed the result as pounds of breech face pressure. In any event, the following table represents some relative measurement across several of the chambers available for this gun.

Marlin Cartridges - Big Bore or 336 Versions
Cartridge    Chamber Pressure     Lbs.\Breech Face Pressure    % +/- .45-70 SAAMI
.30-30              42,000                     5278                             +2%
.35 Remington       33,500                     5002                                  -3%
.44 Magnum          36,000                     5252                                  +2%
.444 Marlin          42,000                7287                            +41%
.45-70 SAAMI       28,000                     5152    
.45-70                   40,000                 7359                             +43%
.45-70                   45,000                 8279                             +61%
450 Marlin             51,500                 9475                             +84%

[securitysix]

Dunno to be honest but this whole mess seems a round about way for folks to try to justify using rounds in the Contender and to some extent the NEFs that the factory and most all after market barrel makes have said are not suitable.

That may be the case for many people.  In my case, it's partly academic curiosity.  That said, if a guy is going to try to make wildcats (which I may never do, but then again, I might, who knows?), it would be handy to know what his platform can and can't handle. 

If a guy had a maximum allowable backthrust number, he could calculate his maximum allowable pressure (using the backthrust maximum and the case head size as givens, a little basic algebra will get you the pressure required to hit that backthrust maximum).  Granted, most people don't have pressure testing equipment, but a guy could look at the pressures the parent case runs at with particular powders and have an idea of the ballpark he's working in, I would think.