Author Topic: Max load in 7mm TCU reached? (Read 655 times)

Crashnrondo · « **on:** November 07, 2005, 11:50:53 AM »

I'm working up some new loads using H322 and Nosleer 120 BT's, I started at the low end and worked up to 28 grains(28.5 is max and I don't like using max loads). The load was very slightly compressed, when I fired these I noticed more recoil and the action was hard to open. It would break but you would have to force it to open the rest of the way to extract the round. 27 grains didn't do this but it gave the same velocity as my other load. My guess is 28 grains is the max for "my" gun, I didn't see any other signs of pressure, only hard opening. Any thoughts?

Lone Star · « **Reply #1 on:** November 07, 2005, 12:51:43 PM »

If the action is difficult to open, then that load is already OVER the maximum acceptable pressure. With a cartridge as small in diameter as the 7TCU, the pressure of that load must be very high to stretch the action. Back off at least one grain, and two would be better. For reference, Speer calls 27.5/H322 as max with their 120-grain bullet, Hornady says 27.0/H322 is max, and Hodgdon says that 26.0/H322 is maximum. Your gun is telling you that with your components, 28 grains is too much!

The force applied to the T/C frame is approximated by multiplying the chamber pressure by the crossectional area of the interior of the case. The smaller the case, the higher the pressure needed to stretch the action. For example, to produce 5000 pounds of thrust on the breech, the 7TCU would need about 60,000 psi of chamber pressure. A .45-70 on the other hand would need to produce only about 30,000 psi chamber pressure. Size matters.

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braud357 · « **Reply #2 on:** November 07, 2005, 01:06:52 PM »

While primer appearance is a good indicator of pressure in most action types - it is NOT a good way to indicate excessive presure in a TC. Looking at the loading manuals I have - you were at least 1 grain OVER MAX ! Firewalling any cartridge is not a good idea - I did this years ago with a 30 Herrett - stretched the frame ! Conventional wisdom advises sticking to the published data. The extra velocity is definitely NOT necessary or worth it !

Reed1911 · « **Reply #3 on:** November 07, 2005, 02:59:07 PM »

Braud,

I believe he showed that he was not trying to hotrod the case or load. No offense, but please don't jump on folks throats when not deserved.

I agree that 28g is over pressure, I would actually back off to 26.0-26.5 and check the accuracy. If it is decent, try playing with your overall length if it needs improvement. Other than that, consider another powder maybe, or changing primers can sometimes help.

Steve P · « **Reply #4 on:** November 07, 2005, 04:08:41 PM »

I believe my accuracy with the 7TCU 120s started to deteriorate after about 25.5 grains of H322. I went to a load of WW748 instead as it kept up the accuracy. Try dropping a few grains and working back up. Don't forget to use WSR primers. You will not be sorry.

Steve

Crashnrondo · « **Reply #5 on:** November 08, 2005, 04:10:12 AM »

Thanks for the replies, this was my first and hopefully last experience with over pressure in a T/C. Not really a excuse but the Nosler #5 manual for the 7mm TCU lists 26.5 for a starting charge and 28.5 for the max, I would guess it's a misprint with what everyone has said, also, I don't pioneer my reloading, I always stay below listed max because my eye's, face and hands are to important to me. I should have known something was up though when I loaded these because it also lists a 98% case fill at 28.5 grains and it filled it to the top after tapping the case at 28 grains.
Thanks for the help-Ron
On a side question, did I do any damage to my frame firing 5 rounds like this? It still locks up tight like it used to.

karbo · « **Reply #6 on:** November 08, 2005, 05:36:41 AM »

The statement about going by published load data is a good one, but it's a good idea to compare different sources, as some data are more optimistic than others. For example: In my Hodgdon reloading manual, the starting loads for 6mm TCU are hotter than the max loads shown in my Sierra data book. I can't even use the Hodgdon starting loads, without excessive pressure. I'm not sure what kind of barrel they tested this in, but they're getting 300fps more than I am.

Lone Star · « **Reply #7 on:** November 08, 2005, 06:52:33 AM »

Quote

...the Nosler #5 manual for the 7mm TCU lists 26.5 for a starting charge and 28.5 for the max, I would guess it's a misprint....

I doubt it is a misprint - Nosler has used that charge as max in the 7TCU since 1989. It is actually a good example of why owning a single loading manual is a bad idea. Why is the Nosler data higher than others? Compared to Hornady data (27.0 grains maximum versus 28.5 grains in Nosler):

Nosler............Hornady
Fed 205.........Rem 9 1/2
WW case.......Frontier case
Lilja barrel.....T/C barrel
9" twist.........10" twist
N/A loa.........2.550" loa

Add in the differences in bullet hardness and powder lots and it is not surprising that the maximum load data is different. One potential key difference is the barrel - T/C barrels have very wide lands, increasing engraving pressures while Lilja barrels have rather narrow lands. And we have no idea of the throat lengths in either barrel.

Quote

On a side question, did I do any damage to my frame firing 5 rounds like this?

I seriously doubt you permanently damaged the frame with just 5 shots. While the original pre-eazy open frames were softer and did permanently stretch, newer frames are stronger and you should be just fine.

Crashnrondo · « **Reply #8 on:** November 08, 2005, 07:07:51 AM »

Lone Star, point taking on different variables, thanks for all the info. One more question, is a sticky extraction the first thing to look for on a T/C for over pressure or something else? Thanks again-Ron

braud357 · « **Reply #9 on:** November 08, 2005, 02:05:20 PM »

To Reed 1911 - I meant no offense - this is why the written word is so often misunderstood. The point that I was trying to illustrate is that in most instances, there is really no extra performance advantage in using max loads. The extra few FPS are not worth the extra wear and tear on components, guns, and shooter. Try reading a bit closer next time before passing judgement - I have ruined a Contender frame from long usage of max 30 Herrett loads, but - boy did it shoot ! (until it loosened up!)

Steve P · « **Reply #10 on:** November 08, 2005, 04:05:41 PM »

Lots of my TCs barrels, the empties just fall out. The TCU family has a habit of smoking the necks when they are fired due to the oversized throats. They have to be pulled out. I usually notice flattening or cratering primers as my loads get a little hotter. Most of my barrels don't need a hot load to be accurate.

Steve