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preasure problems with 7 ultra

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Unread 06-15-2009, 03:06 AM
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Join Date: Apr 2005
Location: Alaska
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Re: pressure problems with 7 ultra

Yes, excess case neck length can increase pressure if the case neck bottoms out on the throat in the end of the chamber and consequently gets swaged down onto the bullet - because the bullet is then pinched tightly into the case and can't blast free as easily at the moment of ignition. But measure the case neck length in the chamber before you trim them back too far. Don't know what model rifle you're working with, but my two Tikka T3s both came with 0.050" excess neck (above and beyond maximum case length) reamed into the chambers from the factory. They probably do this as a safety precaution so reloaders don't blow up their rifles if they fail to keep the case necks trimmed back. As a consequence I only trim to even up the case neck lengths. My Tikka chambers are both 7mm Rem Mags.

That barrel may have the problem Michael expressed. He's gone through two 300 RUM barrels and has convincingly demonstrated the throat erosion & increasing pressure phenomenon. He shoots a heck of a lot and keeps detailed records. Like he said - the 7mm RUM will likely erode throats faster than his two 300 RUMs did.

Keep us posted? I'd like to hear how this turns out as you investigate the cause(s).

Last edited by phorwath; 06-16-2009 at 03:50 AM.
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Unread 06-15-2009, 11:36 PM
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Re: preasure problems with 7 ultra

I also should mention that my throat had not moved any, rather it was just very cracked and burned. This is where the friction came from. This was seen through a bore scope. At 200 rounds, you may not see much if any throat movement yet but if you scope it, you will likely see serious cracking and burning in the throat area.

Interestingly, I have had 2 300 RUM barrel demonstarate the same things, one was a cut rifled barrel and the other was a button pulled barrel. The button pulled barrel showed these signs at a MUCH slower rate and not nearly as severe. The cut rifled barrel showed sign early on and in dramatic fasion.

The ballistics of the RUMs are impressive but these come with a hefty price.

It could still be something else but my money is on the throat.

My solution was that after things settled in was to reduce the loads a bit. I didnt like doing it because I was shooting a RUM for its speed and power never the less, I was forced to. I ended up shooting 180's at 3250 instead of the common 3350-3400+. This gave me some stability and consistency.

If your barrel turns out to be the same, you may consider reducing your loads a bit. If this is just starting to manifest itself I would suspect you are using a button pulled barrel?? If so, you should have some accurate life left by reducing your loads. Another concideration is to have it cut back a bit on the breach end and have it chambered in 7mm WSM. This will offer you the same bolt face and more barrel life, less recoil and evry bit as good if not better accuracy and certainly more consistency.

I hope that helps some.
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
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Unread 06-16-2009, 04:30 AM
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Join Date: Jun 2007
Location: Hawkes Bay New Zealand
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Re: preasure problems with 7 ultra

Originally Posted by harfman99 View Post

I was doing some load workup this last week and found the one. The load 180 berger vld 84 grains of retumbo fed 215 primers. fired and annealed rem brass. The load gave me 3283 avg with a average diff of 8 fps and gave me .25 moa at 600. The brass was handling it very well no signs of excessive pleasure at all and i couldn't have been happier. I went out with my brother Friday evening and had problems with over pleasuring cases. It was only about 65 degrees when we were out and only about 10 warmer than it was when i did the load workup. so temp shouldn't have been the problem. All cases had a had a normal going through neck and mouth cleaning and partial neck sizing, primer pockets also cleaned. the powder scaled out to the 10th of a grain and from the same jug.
bullets were all set out to the same leignth. the bullets are just touching the lands.

I don't know what i am doing wrong to cause such a big diferance in preasure. Would it be the barrel the brass of something i am overlooking.
also the problem wasn't continious with three over preasuring and the next round not and several more after that and back and fourth. I put 12 rounds down range that nite and kept the shots spread apart to keep the barrel cool.

any thing else to look at would be greatly appreciated.

3284 fps with a 180gn Berger out of a 7mm RUM is over the edge unless you're running a 30" barrel? Out of the usual 26" barrel even in the sloppy factory chambers with the long throat 3200fps is about maximum.
If you're right on the edge pressure wise, the carbon build up and slight throat roughening as you fire progressively more rounds can push you over the edge, especially if the temperature is warmer as well.
Case length can be an issue, but most factory RUM chamber lengths are 2.875" or longer. Even a tight custom chamber will usually take brass 2.870" long with out spiking pressure. Don't trim until you've actually measured your chamber length, or you may trim too much.
Don't be put off the RUM, so long as you don't run it over the edge or expect to fire large numbers of rounds through it in rapid succession, it'll give superb long range performance with managable recoil that is hard to beat. I have built many 7mm/404's and 7mm RUMs and have never found them a problem to load for. I use mainly 2218/H50BMG powder and anywhere from 99 to 104 grains with the 180 Bergers and 175 SMK's depending on throat length. I have always got good barrel life when used in big game hunting situations. They are not varmint or match rounds, but a 180 Berger at 3200 fps out of a 26" barrel is a pretty darn good long range hunting performer!
NZ Longranger
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Unread 06-16-2009, 02:28 PM
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Join Date: Mar 2008
Location: South Dakota
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Re: preasure problems with 7 ultra

the barrel is 31" and the load should be right in mid range for the data i received from Berger. My plaster cast did not come out very well in such a long chamber and was unable to get the measurements I wanted. I will call down to Shilen and ask if i can get a measurement off the reamer. I hate to try to do new load development trying to keep the round count down. I went ahead and cut the cases down to a standard 2.850 wich will take out that variable. I am also going to do a partial full leignth sizing being that the cases do chamber a little stiff. I have also lapped the heck out of the bore using jb bore paste hoping that this will help and smooth out the throat. the weather is supposed to cool down in the next couple days and will hope to get out to the range and see if any of this has helped.

thanks for all the info it is greatly appreciated.


Last edited by harfman99; 06-16-2009 at 02:34 PM.
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Unread 06-16-2009, 03:12 PM
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Join Date: May 2008
Location: South of Canada and North of Wyoming
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Re: preasure problems with 7 ultra

I think I would have it bore scoped if that is possible...
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Unread 01-31-2010, 03:25 AM
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Join Date: Jun 2005
Posts: 167
Re: preasure problems with 7 ultra

max case length is 2.850. Trim to 2.840
Also, was your first firing on FL sized brass (which was ok for pressure), and the next firing on partial neck sized brass (when pressure appeared)? If so, maybe you should look at working up the load in exactly the same way you intend to size your brass. If you intend to use partial neck sizing, then work up your load using partial neck sized brass. Also, work up your load in the warmest weather you plan on shooting in. You might lose a few fps in velocity when its cold out but you wont be running into pressure problems.
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Unread 01-31-2010, 06:10 AM
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Join Date: May 2007
Location: Taranaki New Zealand
Posts: 186
Re: preasure problems with 7 ultra

I have now owned and reloaded for four 7mm Ultra's and I am pretty sure I know what your problem is- the powder charge is too light- way too light and your cartridge is suffering directional ignition/ secondary detonation or whatever the heck you want to call it. It the same problem as occurs with a light charge of W296 in the .44 magnum.

You really need to be using a minimum 90 grains Retumbo to alleviate this problem, more if you are using a long COAL. Velocity should either drop or remain the same, strange as it sounds. You could switch to H50BMG but I find it burns the throat out in as little as 150 shots. Michael, who has been posting on this thread, is right on the money about throat life. The 7mm RUM (and STW) is problematic which is why there is a resurgence of interest in the 7mm-.300 Win mag wildcat and variations.

Shoot your rifle, enjoy it while it lasts and be happy to see the back end of the 7mm RUM when its worn out.
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