Gunsmithing New bbl making crazy velocity/pressure at low charge weights??

[chefcam864]

I have a new 28", 1:10", 6 groove Lilja bbl chambered in 308 win on my TR gun, and it behaves strangely... The rifle makes big velocity at charge weights that just shouldn't make those kind of numbers. Furthermore, it goes from no pressure signs to blowing primers VERY QUICKLY.

These are chrono results, from the gun in question, taken with two different magnetospeed chronos

185 Berger LRBT (ws2 coated) 0.010" jam
43.1 Varget
Lapua ( necks turned to 0.014" for loaded round OD of 0.336", chamber neck=0.339")
WLR
Avg: 2,726
SD:4

185 Berger LRBT(ws2 coated) 0.010" jam
41.8 H-4895
Lapua (necks turned to 0.014")
WLR
Avg: 2,686
SD:7

175 SMK (ws2 coated) 0.015" jump
41.5 4064
LC ( necks turned to 0.014")
WLR
Avg: 2,704
SD:9

It usually takes 44.5-45.0 Varget to get that velocity with that bullet/powder combo. The velocity jumps to 2,750 with naked bullets. I blew primers at 43.5.

Furthermore, when testing with 4895, I shot 10 rds. @ 42.0 and only had a slight ejector mark on 2 cases, and next to zero case head expansion. The primers had a nice radius and were not "cratered" at all. I the shot 5rds at 42.3 and blew a primer out on the 5th round. All of those cases exhibited about 0.0015" case head expansion. This was shot 15 mins after shooting 42.0. Something is amiss, but I don't know what. I'm going to call Dan Lilja and see what he thinks, but would also appreciate any input.
I have plenty experience with reloading in general, and a good bit of experience with all the powders & bullets in question(especially 4895), and those numbers just do not jibe with my previous experience at all.

 

[Hookturnr]

I have a Hart barrel in 300rum that did this same thing with Retumbo...stuck cases at 88gr. if I remember correctly. Couldn't find anything wrong dimensionally with the chamber, switched to H1000 and the case sticking went away but still gets crazy speed for the charge weights. I'm only using 86.0 gr. of H1000 and getting a 208 AMAX to 3060fps. for best accuracy. I can't get anywhere near max listed charge weights but I can get a ton of speed. I mic'd the bore and found that the bore was on the tight side as well as the chamber being absolute SAAMI minimum. I figure it's a combination of those factors that is getting me the speed/charge wt. I'm seeing. It was frustrating starting out but has turned out to be hammer of a 1/2min. gun.

 

[bohem]

I suspect that the first reply hit on a key factor, bore dimension. Adding to that Lapua brass is somewhat thick and low internal volume and WLR primers have shown themselves with my testing to be very hot.

Sounds like a fast, powder efficient barrel. Lucky!

 

[chefcam864]

I have no way to measure the bore dimensions. My smith said he checked the bore with a gauge pin, and it was what it's suppose to be, but he said he had no way to check the groove diameter. It's a minimum SAAMI spec chamber, except it has a tight neck. He has chambered identical Lilja's with the same reamer on many, many rifles, but this one is definitely different... Damn thing shoots great! It's just tricky to develop loads for because it can go from zero to on the boil in 3/10's of a gn. I found a nice load with H4895 & 185 LRBT's that shoots just over .5" @200 and just about 1" at 300, (that's with my sorry butt shooting)so she's a winner. Albeit an unforgiving winner.

 

[buffybuster]

Furthermore, when testing with 4895, I shot 10 rds. @ 42.0 and only had a slight ejector mark on 2 cases, and next to zero case head expansion. The primers had a nice radius and were not "cratered" at all. I the shot 5rds at 42.3 and blew a primer out on the 5th round. All of those cases exhibited about 0.0015" case head expansion. This was shot 15 mins after shooting 42.0. Something is amiss, but I don't know what. I'm going to call Dan Lilja and see what he thinks, but would also appreciate any input.
I have plenty experience with reloading in general, and a good bit of experience with all the powders & bullets in question(especially 4895), and those numbers just do not jibe with my previous experience at all.

Those "lite" 42.0gr loads weren't light. Even though they didn't show much pressure signs, they were at or likely above the SAMMI maximum allowable pressure. Primers generally don't show pressure signs until near 70Kpsi. Velocity comes from pressure...... you're crazy velocity is coming from pressure and a long barrel. The barrel might be on the tighter side too.

 

[Rooster721]

I'd bet my bottom dollar it's simply a tight chamber. Brother & I had two rifles done identically (same barrel, same caliber, same action.. same reamer even!) But, barrels were chambered about 2wks apart from each other* We brought them home, started testing loads and saw the same thing. One rifle shot as it should, velocity/charge everything was as we expected.. the second one fired the same load/bullet 175fps faster!!! (but was pressured) ...once we narrowed it all down, the chamber wound up being a matter of about 2thou tighter. Variance in machining at different intervals. That's it.

Only way around it is to load accordingly to that chamber. And on the bright side, tight chamber means less brass flow/stretch (means longer case life) and inheritantly more consistent accuracy as well. Full sizing that chamber was bumping so slightly that it's basically good as neck sized only* (in a sense anyway, when using it's specifically fired brass when re-loading)

Your situation sounds very similar.

 

[chefcam864]

The chamber is tight, for sure. (It was designed to be.) I can't extract brass that was fired in another gun, even if it was small base sized. It's kinda weird, it'll feed just fine, but when I go to open the bolt, I have to tap it back to get it to open. I know what you're all thinking, but no, it's not pressure. After they'd fired in my chamber and small base sized they work fine.

When you say your chamber was 0.002" tighter, which dimension are you referring to?

 

[Rooster721]

Headspace, in the case of fired-case measurements.. Basically, if my brother takes a case I fire through mine and tries to cycle it into his (like you are saying) it won't go. Gotta be fully resized and bumped back to fit. When measuring fired cases, his are fireformed roughly 2thou shorter than what mine are through my chamber-- hence, +/- the 2thou tighter statement. That said though, it probably wouldn't be too far-fetched to say the chamber diameter (in yours anyway) must be some degree tighter as well to be causing the little bit of expansion in the case-walls to cause issues... are there marks that could hint toward rough-spots inside that chamber that might need polishing? If re-sizing the body is needed to provide clearance, maybe a guy should check that the expanded cases aren't in-fact hanging up on some imperfection inside that chamber ??

 

[chefcam864]

Headspace, in the case of fired-case measurements.. Basically, if my brother takes a case I fire through mine and tries to cycle it into his (like you are saying) it won't go. Gotta be fully resized and bumped back to fit. When measuring fired cases, his are fireformed roughly 2thou shorter than what mine are through my chamber-- hence, +/- the 2thou tighter statement. That said though, it probably wouldn't be too far-fetched to say the chamber diameter (in yours anyway) must be some degree tighter as well to be causing the little bit of expansion in the case-walls to cause issues... are there marks that could hint toward rough-spots inside that chamber that might need polishing? If re-sizing the body is needed to provide clearance, maybe a guy should check that the expanded cases aren't in-fact hanging up on some imperfection inside that chamber ??

Cases fired in my chamber are +0.001" over what new, unfired cases measure. There is nothing to indicate any imperfections in the chamber either. I know all of these loads are "hot", but if you spend enough time around F-TR shooters, you will soon find out that nearly everyone is pushing hard.

What is puzzling, is that 4 ppl I shoot with use the bbl's with chambers cut with the exact same reamer( not one like it, but literally the same reamer) and it takes between 44.5 & 45.0 Varget to get 2,750 with a 185 LRBT. It's puzzling, but it shoots...