bore configuration and pressure

[spitballer]

I wonder if I might please get some feedback from anyone else who has practical experience with polygonal rifling in a long gun: this barrel I'm using seems to be much less forgiving now that it's broken in. My reloads are showing pressure signs just as soon as they are even slightly past optimum. BL-(C)2 seems to be more forgiving in this regard but H4895 and Varget are just too much, load data notwithstanding. Question: in a standard, rifled bore do the sharp corners along the lands allow a certain amount of gas to escape and help to regulate excess pressure or something? I don't recall my original rifled barrel being nearly so finicky as this one. Any anecdotal data appreciated. I also understand why there could be hidden fouling build-up in this case, and assuredly I'll thoroughly check this out and solve it if necessary, but the inflexibility in regard to pressure is profound - is this typical for smoothbores?

 

[tkroenlein]

Disclaimer: I have zero experience with polygonal rifle barrels.

With that said, there are so many variables in rifle barrels that affect pressure, that it's usually summarized by saying "no two are the same."

Typical "square cut" rifling does not by nature bleed off pressure. The differences that do have a pretty large affect on pressure are bore/groove diameter, rifling pattern and twist, throat length and diameter, lead angle and chamber dimensions.

My guess is you have a barrel that has several more of these attributes that tend to increase pressure than your old barrel.

Have you chronographed anything in the old barrel for a baseline and the new barrel for comparison? You might find the new barrel is faster even though the charge weight isn't as high.


Sent from my iPhone using Tapatalk

 

[spitballer]

Typical "square cut" rifling does not by nature bleed off pressure...

...okay I didn't think they did, with normal operating pressures, but it's good to get independent agreement on this. The other possible factor might be fouling buildup of one sort or another. I've deliberately avoided the bronze brush with this barrel and may have a buildup of Teflon or other material not affected by bore solvent. It might be worth a pass or two with the brush. But before I use gnarly solvents or a heat gun to do away with possible buildup I'm going to try to work with it first. So far pressure has shown up on the second shot like it normally does so maybe buildup has simply made tolerances tighter and less forgiving and there's not much more to it than that. I wouldn't have even bothered to start a thread if it hadn't been for the pierced primers yesterday, my first. Thanks tkroenlein for your valued input.

 

[mdi]

Interesting thread. What high pressure signs are you seeing at what level loads? Could you give some examples (what caliber?) mebbe someone can compare their "standard rifling" loads to your "poly rifling" loads?

 

[spitballer]

Could you give some examples (what caliber?)...

Oh...sorry, this would be a .223 Rem 700 bolt action with 12" twist. Normally I use 55gr bullets exclusively but I had some 64gr varmint bullets on hand and I thought I'd use them. It was when I used the heavier bullets that I ran into trouble, even though I was well within load data specs. I hesitate to give specific figures because I don't want to cause undue concern for guys who depend on this data, but when old-timers tell us that load data serves only as a guideline, they're not kidding! I will say this, with H4895 and the 64 grain bullets I got a pierced primer within a half grain of the published starting load. Admittedly the longer 30" barrel may have been responsible for producing excessive pressure. Fortunately I got lucky and my bolt is still in good shape but I'm going back to my 55's with about 25 +/- grains of BL-(C)2. For some reason the BL-(C)2 seems to be a little more forgiving and 5-shot groups tighten up at lower velocity. FWIW the mag primers seem to be a little stouter too.