Need help troubleshooting 300 RUM pressure issue.

[MontanaRifleman]

Specs
Trued and nitrided 700 action with one piece PTG bolt (also nitrided)
Bedded in an HS precision stock
27" 5C Broughton Barrel with SSG brake, also nitrided

All velocities were from a Magnetospeed.

OK, about a week ago I loaded up cases from 94 gr to 103 gr of RL33 pushing 230 Hybrids along with work up loads for my other 2 rifles, 6-284 and 6.5 WSM. Went out to the range and worked up all the way to 103 gr. At 103 gr I had a slight bolt lift (no ejector mark) with an MV of 3192. The 102 load show no signs of pressure and MV was 3162. Ambient temp was about 23*.

Today I decide to do some temp regression testing with my 3 newest rifles, the 300 RUM being one of them. The plan is to load here at the house and shoot out the back window down into a wooded ravine using a big cottonwood as a backstop. Temp inside is about 75*. I accomplish the shooting with the 6-284 and 6.5 WSM without any hitches. 4 shots each with the loads I identified as max from last weeks shooting. Velocities were a little higher but no pressure signs except one slightly stiff bolt out of 4 shots from the 6.5 WSM.

Then I chamber a 230 Hybrid with 102 gr of RL33 and fire. Very heavy bolt and blown primer, MV 3215. Very surprised at this. I go to the bench and load one with 101 gr. Chamber and fire. Heavy bolt lift and powder ring around the primer and big shiny ejector mark, MV 3219. Go back to the bench and load one with 100 gr. Same exact signs as the 101 load, MV 3160. Back to the bench and load one with 99 gr. Almost the same signs as 100 but slightly easier bolt, still heavy though, MV 3186.

I decide to give the bore a quick scrubbing. So I give it a few passes with nylon brush and BTE. Then run half a dozen wet patches through it until powder and copper is gone. Then patch it dry. I load up another 99 gr round and this time, very easy bolt lift and no signs of pressure, MV 3101... hmmmm... so I load one with 100 gr and boom, back to heavy bolt, powder ring around the primer and big shiny ejector mark, MV 3207. I go back down to 99 and boom, all the bad signs and MV 3192. Then I go to down to 98, and boom, all the bad signs, MV 3164.

Then I try another quick cleaning and load up 98 again. This time i still get the bad signs, MV 3148. Bolt lift isn't real heavy but it's a bit heavy with ejector mark and powder ring. So I load 97 gr and still get pressure signs with an MV 3121. That's it.... I throw in the towel

Something is not right here fellas and if you can help me figure it it out I would be very grateful.

And yes, I did check the accuracy of the scale (Chrgemaster) against my 10-10. It was right on.

 

[Augustus]

Are you loading new brass each time or are you loading the same pieces multiple times?

 

[MontanaRifleman]

Are you loading new brass each time or are you loading the same pieces multiple times?

The brass used for work up a week ago was virgin FL sized. The first round of 102 gr fired tonight was the same. The rest were the same lot 2nd firing FL sized.

 

[bigngreen]

Not cool!! Having the Magneto on when it was happening seems to be showing something is making pressure to get that same velocity, I don't think I would be pulling the trigger on it till I checked the head space again and had a little look in the barrel!
I shot some R33 today as well and the results were interesting, I'll PM you what I saw, it may be relevant.

 

[MontanaRifleman]

Not cool!! Having the Magneto on when it was happening seems to be showing something is making pressure to get that same velocity, I don't think I would be pulling the trigger on it till I checked the head space again and had a little look in the barrel!
I shot some R33 today as well and the results were interesting, I'll PM you what I saw, it may be relevant.

Yup, something is causing pressure and it seems with every trigger pull it gets worse... really scratching my head

 

[Augustus]

The 300 Ultra Mag can grow brass quickly if the shoulder is bumped back excessively during resizing. This can result in the case mouth clamping down on the projectile when chambered. It is inevitable that pressure will increase when the Rd is fired.

You may have already thought of this but just try to push a bullet into the mouth of a freshly fired piece of brass that showed unexpected signs of pressure. If you are having growing pains the bullet will not want to go in the hole.

You may have marginal clearance in the neck, some of the brass may be thicker than others causing the clearance to be inconsistent. Measure the necks of your freshly fired cases and those of the loaded rds. The loaded rds necks should be .002 or so smaller than the fired ones.

Those are the simple fixes, if one of these are not the problem you may have some very bad juju creeping upon you.

 

[MontanaRifleman]

The 300 Ultra Mag can grow brass quickly if the shoulder is bumped back excessively during resizing. This can result in the case mouth clamping down on the projectile when chambered. It is inevitable that pressure will increase when the Rd is fired.

You may have already thought of this but just try to push a bullet into the mouth of a freshly fired piece of brass that showed unexpected signs of pressure. If you are having growing pains the bullet will not want to go in the hole.

You may have marginal clearance in the neck, some of the brass may be thicker than others causing the clearance to be inconsistent. Measure the necks of your freshly fired cases and those of the loaded rds. The loaded rds necks should be .002 or so smaller than the fired ones.

Those are the simple fixes, if one of these are not the problem you may have some very bad juju creeping upon you.

I only size the cases back enough to fit snugly in the chamber with an easy bolt lock. The fired necks are are about 4 or 5 thou larger in diameter than the seated necks. The bullets slip easily into these fired necks. A little more easily than previously fired necks from mild barrel break-in rounds. Those necks would hold a bullet but it could be easily pushed in and out. There might be something with the neck chamber causing this. Maybe some residue from cleaning or whatever.

Thanks much for your thoughts

 

[Augustus]

I would run a few rds with Retumbo and see what happens. Your primer may be struggling with the RL33.

 

[Dano1]

This is just a thought, so take it for what it's worth,

But I had a similar experience with my 6.5-06AI when developing loads using RL-22 and there was quite a difference in temperatures from when I developed the loads (about 75 degrees) to when I was shooting them later (about 90 degrees) the loads that were safe in the 75 degree weather blew primers and were hard to extract at 90 degrees, they were right on the line of being too hot at 75* but felt I was safe.

I share this for 2 reasons, 1) the 40 degree shoulder hides the pressure quite a bit in the Ackley case, as does the very heavy case with the RUM cases. So when coming up to pressure it can and does come up quickly even in 1/2 - 1 grain increments even in such high capacity cases. I load for two different .300 Ultra Mags a we Use Retumbo and 180g bullets, While Retumbo isn't the same powder as RL-33, We are only using a 99g charge and are with in 1g max for 1 rifle and 1/2g max for the other with the 180g bullets (Nosler Accubonds). Maybe too much powder?

2) Maybe RL-33 is temp sensitive like RL22 and can't handle such extreme temp swings......

Just my thoughts and perspective.

Hope this helps,

Dan

And

 

[Michael Eichele]

No need to scratch your head. I have an answer for you but your not going to like it.

I recommend having a look at the throat and first few inches with a good bore scope. That'll show you everything you need to know.

 

[MontanaRifleman]

Thanks for input Dan, When the 102 gr load over pressured on me I thought maybe it was the temp diff. But when the it continued to over pressure all the way down to 87 gr, I figured something is going on with the rifle. After I cleaned it the first time and got low velocity with no pressure signs at 99 gr and then spiked up 106 fps when increasing to 100 with more big pressure signs and then backing down to 99 and still seeing pressure with a velocity 91 fps greater than the 99 gr load I had shot just 2 shots previous, I figure there is something going on in the chamber or bore. Who knows.

Something to note is I used the same powder from the same jug to shoot the other two rifles. Both of them have 35* shoulders and the RUM a 30* shoulder. I also shot the max loads from last weeks workup at lower temps also. The load in the 6.5 WSM was 74 gr (very compressed) and i did try a round at 74.5 prior to loading and shooting the 74 gr loads. It showed slight pressure, so I backed back down to 74. Velocities shooting 140 VLD's were,

3441
3431
3453
3457

for an extreme spread of 26

The 6-284 velocities shooting 105 Hybrids were,

3513
3538
3521
3536

for an extreme spread of 25

High velocities and max pressures with no hiccups.

33 is somewhat temp sensitive, but I don't think that is the problem in the RUM

I also loaded Retumbo and 180 bullets (E-Tips and CEB's) in my previous 300 RUM Sendero. My max was 98 gr.

Thanks again for the input

 

[MontanaRifleman]

I would run a few rds with Retumbo and see what happens. Your primer may be struggling with the RL33.

I will probably try that. I plan to take it out to the range tomorrow and redo the 1 gr workup in outside ambient temps (forecast to be about 20 colder than last week) and see what happens. I also ran up a srting with Retumbo and 180 CEB's and RL17 with GSC 177's which Ill also repeat.

 

[MontanaRifleman]

No need to scratch your head. I have an answer for you but your not going to like it.

I recommend having a look at the throat and first few inches with a good bore scope. That'll show you everything you need to know.

Good idea Michael. That's just what I plan to do Monday if I don't get this cleared up before hand.

 

[Michael Eichele]

Mark,

I had two different custom 300 RUM barrels do the exact same thing. Both started doing it early in their life and one did it less than 70 rounds. It seemed like every time I pulled the trigger the velocity went up and the bolt got stickier. I kept backing off charges and velocities still went up. I fought and fought with it for a couple hundred rounds and it settled in to a point but never stopped settling down completely. After a certain point, instead of dropping charges every few rounds, I only had to drop them every 50 rounds or so. When it was all said and done, the only powder I could get to work in one was H50BMG under the 178 amax and 180 accubond. In the other I was down to 80 grains of H1000 under the 178 amax or 92 grains of H50BMG under the 208 amax. All were reduced velocities but still showed signs of fairly high pressure.

Shortly after it started happening on my second barrel, Mike Rock told me it was from severe throat erosion and severe heat checking. He said those will actually increase friction on the bullet dramatically and that is where my high pressure was coming from. A trip to the smith and look through the borescope confirmed his statements. It was bad. Really bad.

It can happen VERY fast in a barrel chambered in 300 RUM. Especially if you run top end loads. In my experience, top end loads in 300 rums even without pressure signs initially, kill barrels and fast.

I know thats not what you were hoping to hear and I hope I'm wrong but that's most likely the problem.

Factory barrels can sometimes hold up better IMHO. They're often hammer forged and subsequently very durable and so rough they can actually improve before going downhill. With a top end fine custom barrel, they have ONLY one way to go. That is downhill. You can't typically improve a good custom bore. They're their best for only one shot. That's their first. They start going downhill after that. The hotter the caliber and the harder you run them, the faster they decline. It's not at all uncommon for the 7mm RUM and 300 RUM customs to go south short of 100 rounds. I get PMs occasionally about how fast one went south. I used to preach my feelings on this matter a lot and caught a lot of flame for it here. Guys not wanting to catch flame would pm me and say yup! You were right!

In this case I truly HOPE I am wrong.

M

EDIT: I almost forgot, make sure there is no carbon ring present either. I'm just guessing here but it doesn't sound like you have enough rounds down it to be a carbon ring but a carbon ring will generate a lot of extra pressure and if not corrected quickly, can kill your barrel as well.