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results of barrel setback and rechamber

 
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  #1  
Old 10-03-2003, 06:18 PM
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Join Date: Jul 2001
Location: Walla Walla, WA
Posts: 174
results of barrel setback and rechamber

I wanted to let you all know the results of my barrel set back and rechamber. The barrel is a Lothar Walther and it was/is chambered for 300 Win Mag. The original gunsmith who installed it gave it a longer throat than I wanted (it was 0.300 long and was parallel at 0.3085 diameter). The action is a Winchester Model 70 Classic Stainless with the pre-64 type action. It has a 3.6 magazine box. It has a synthetic Rimrock sporter stock and a Swarovski 3-9 AV scope and weighs 9.3#. It is my climb the mountains big game rifle. When I sent the rifle to the smith who installed the new Walther barrel I sent him a dummy round to set the throat length but he made it too long. It shot well initially with the 220 SMK and the 200 Partition but after about 1000 rounds the throat had lengthened and accuracy was not that good with that much jump to the lands. So I sent it to Greg Tannel to have him set the barrel back and rethread and rechamber. He did an excellent job, giving it a neck diameter of 0.002 over loaded round neck diameter, a 0.3083 parallel throat of the specified length of my dummy round. I wanted to shoot the new 200 grain Nosler Accubond bullet.

When I got it back from Gre-Tan Rifles I started with H1000 but then tried RL25. In the previous chamber with the longer throat I was using 79 grains of RL25 with the 200 Partition for 3030 fps but this new chamber with the shorter throat requires only 74 grains of RL25 with the 200 Accubond to get 3030 fps. In my 7mm Rem Mag the 160 Partition and 160 Accubond use the same amount of powder for the same velocity so the difference in the 300 Mag must just be the tighter chamber.

Anyway I started with 76.5 grains of RL25 based on the fact that 78 grains of H1000 blew a primer. Accuracy was not great so I backed down to 75.5 grains. I started on the lands but 0.050 off the lands has proven to be the best seating depth. Then I tried 75 grains of RL25 with the 200 Accubond at 0.050" off the lands. I shot 4 shots at 250 yards and they landed in a 1.87" group. Then 3 shots at 400 were in 2.46". Then I shot 5 shots at 500 yards and they were in 4.93". Velocity from my 25" barrel is 3070 fps for 13 shots recorded. This load is a bit hot even though the powder charge is low. Incidentally my Swarovski 3-9 AV scope's clicks have a value of 0.28"/ click, not 0.25". It only took 30 clicks up at 600 from an initial zero at 250 instead of the 34 clicks that it should have taken (based on the ballistic coefficient and velocity and elevation).

I then tried 74 grains of RL25 with the 200 Accubond seated 0.050" off the lands. I have shot this bullet in this chamber with 76.5 grains and did not blow the primer out like I did with 78 grains of H1000 so while 75 grains of RL25 is fast it does not seem dangerous. Anyway I was concerned with the high velocity of 3070 fps with the 75 grain charge and since 75 grains looked to be more accurate than 75.5 grains I thought I should try 74 grains. First group at 250 was 1.02". Second group at 250 was 1.72" with all 6 in 1.72" (5 of those were in 1.02"). There was only about 0.7" of vertical for all 6 shots at 250 yards. First group at 400 yards was 3.82" (one shot well to the right of the other two with only 0.75" of vertical). Second group at 400 was 2.11" and it landed in the middle of the first group so I had 6 in 3.82" at 400. Velocity was 3030 fps. I have my load!

Finally I went out to make sure that my rifle was zeroed from field shooting positions, not just from the bench. I shot the 200 Accubond with 74 grains of RL25 load at 250 yards from the prone position using my Kramer Snipepod and sling. Group was perfectly centered and measured 1.7". Then I tried a group from the sitting position with my taller Kramer Snipepod and amazingly shot a 0.76" group that landed in the middle of the first group such that all 6 shots were in 1.7" at 250 yards. So now I have fired 6 groups of 3 shots each (4 at 250 and 2 at 400) with the 200 Accubond seated to 0.050 off the lands and 74 grains of RL25. Average group size is 0.594 MOA. I love this rifle. I first sent this rifle off to Arnold Arms in December of 1998 after wearing out the factory barrel and 4 barrels later gave up on them (none of them shot accurately). Then I sent it to a reputable 'smith but he gave it a longer throat than I wanted. Finally it is perfect after 5 agonizing years. Rufous.
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  #2  
Old 10-03-2003, 07:14 PM
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Join Date: Dec 2002
Posts: 374
Re: results of barrel setback and rechamber

rufous,

"The load is a bit hot." No kiddin... 25" barrel, >3000fps 30 cal 200gr. I'd say perhaps just a little past hot.

I would say if you brought the same load down below 2800 to say just over 2700, it'd shoot terrific. If you don't think it has enough snot to kill something at 5-800 yds, just stand out in the field and have someone shoot you with it and tell em if it hurts or not.

I don't know what barrel diameter you're shooting or how the gun is set up (stock/rest/etc) and I don't want to discourage you. Different setups are designed for different applications and one is not necessarilly right for the other, but, if my 16# gun shoot's a 10 shot 3" group at 400 I'm gonna tear it down and fix something. It's a last place gun at williamsport (reference the standings). I'm taking a little of my own advice this weekend on my 6.5 and SLOWING DOWN. I shoot my 30 cal W/210 at <2800 with a 28.5" barrel and barely a 2" case, (barrel is used for acceleration, not case holder). My guess would be that of the shooters w/30cals and <=30" barrel, 80% shoot under 2850fps and 100% of those who win shoot under 2850fps. I would guess that your gun that shoots 5" at 500 will not keep 10 on a 42x62" target at 1K as my 6.5 that shoots <3" (too fast) at 400 cannot keep 10 in 2feet at 1000. Lots of things happen out there past 500.

Everyone's been there, thinking "this is great", the moral of the story is, it'll do better with some work.

PS, I'd have stuck with the shot out throat and worked with it. Glad to see it improved for ya. Try 66-68gr H1000/R25 just for kicks. Bet it shoots better...
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Old 10-03-2003, 07:52 PM
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Join Date: Jul 2002
Location: Atlanta
Posts: 61
Re: results of barrel setback and rechamber

lothars run fast. super smooth lapping done on CNC machines. a friends 243 was runnin 3050 with 107's in a 24 inch tube. was at max load in sierra expected velocity was right around 2900

[ 10-03-2003: Message edited by: J Schulze ]
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  #4  
Old 10-04-2003, 05:39 AM
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Join Date: Jul 2002
Location: France
Posts: 390
Re: results of barrel setback and rechamber

barrel lapping is made by hand that the only way to "feel" the small defects , and all the match barrel maker hone or lapp barrel by hand
CNC machine ????? perhaps for drilling and profling , perhaps to for buuton rifling ( why not )

Lothar are made by button rifling , internal quality of smoothness is these ame as finish for most barrel maker wich use button rifling as process.
Smooth barrel is not a quality , straight barrel is a quality and to get velocity the only way is pressure so without pressure checking as strain gage or pressure barrel test no real reason to glorify th ebarrel maker

I can get Lothar barrel very easy ( I am a 450 km from the factory ) but I don t use them except carbon steel barrel for fire forming

good shooting and check your pressure load

DAN TEC
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  #5  
Old 10-04-2003, 09:23 AM
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Join Date: Dec 2001
Location: occupied, USA
Posts: 189
Re: results of barrel setback and rechamber

To reverse calculate your scope's adjustments based on what your ballistics "should be" is incorrect. Chances are much greater (exponentially greater) that your load does not fit the curve you've calculated, as there are many variables in the equation (plus whatever margin for error in the ballistic equation/software) that would account for what you observed.

To explain the variation in expected impact as a scope click adjustment error of 0.03" is just not good science -- but it's an easy way out. [img]images/icons/smile.gif[/img]
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