Changing from 7-08 to 7mmBr

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  • Last Post 15 January 2017
karlrudin posted this 13 January 2017

IN YOUR OPINIONS, I am currently considering converting my 7-08 to 7mmBR in a quest to lighten my recoil but hopefully tighten up my groups. What I would like is y'alls input on how the 7mmBR performs, pitfalls or problems with the cartridge. I have been seeing a LOT of winning scores from the 30BR folks. But since I already have a 7mm bore, I was going to try this rout first. All comments welcome. Currently I'm using WW/2% Tin water dropped. A Lyman 7mm 160g #287641 mould. Aluminum gas checks, glued. Lee Alox cut 20% with mineral spirits. Average velocity, 1500fps. My groups generally hover between 3/4-1” overall. Thanks y'all.

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Ken Campbell Iowa posted this 13 January 2017

mr. karlrudin ... i have a spare shilen varmint barrel for a remington 700 in 7br 1/10 short  .... and wouldn't mind playing with a 7-08 if in a quality barrel ...wanna talk trade ??  

i already have a factory barrel in 7-08 .

pm me if

ken

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frnkeore posted this 13 January 2017

Within your existing perimeters, you might pick up .1 to as much as .25 in accuracy by going to the shorter case, because of better loading density. Meaning lower ES, producing better barrel harmonics. It will take some shooting and averaging groups, to prove it out though. I'm not sure whether you'll notice a reduction in recoil though.

The 7BR is quite capable of producing much higher velocity that could result in smaller group, do to less wind deflection. But, that would mean much more recoil. 

I would also, especially at your velocity, suggest one of the RCBS molds. Either the 145 or 168 gr molds, the 145 would reduce recoil much more that a reduction in powder. I have a XP100 in 7mm TCU (15” bbl) and use the 145 gr bullet with full charges of 4895 and 2520 powder. With that combo, I get about what your getting with your rifle now but, at 2000 fps. It does about 5/8"  groups with with 140-145 gr jacketed with the same charges as cast, with the factory stock.

Frank

 

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karlrudin posted this 13 January 2017

More info, load density is what I was thinking about using the 7BR. As it is right now my sweet spot is only using 20g of IMR 4198. Lots of air space left in the case. I have tried several diffent powder changes and it seems to like this one. From HS-6 to IMR4064. My bore does NOT like the lighter bullets because I have a 130g mould also and the best it will do is about 2". As far as the bullet starting off inline, I use a bore rider which is .001” larger than the grooves and my first driving bad is imprinted into the lands also. Thus it should be starting off straight. Along with the fact that my case necks are .003 smaller than the chamber since I turn my necks. I was advised not to make the necks any tighter. This might not be true though, but my thought was on to tight of a chamber to neck clearance thus a bullet release problem. As far as adding more weight to my weapon, this I'm kinda stuck at because I have to stay under 14lbs for shooting in tournaments. And I'm at 13.5lbs. As far as cases go, I have tried Lapua 308 Palma cases with the small primer pockets, but my Remington brass still seems to do better. Thanks for the info thus far.

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Ken Campbell Iowa posted this 14 January 2017

mr. karlrudin..

just some rambling follows :

with your large case ... if you shoot a couple groups in your 7-08 with match jacketed bullets and 2200 fps with ? 4198? ....?tightgroup? ... etc ... would it not seem possible to shoot 1 moa or less with good/better/best cast bullets ? ....

although of course as soon as you might achieve -1 moa with 7-08 you would be looking at the 7 br again ... and yes, those little 7mm br are really cute ...

cast can be a bit frustrating, EH ?   ( g )

 

oh, i have a rcbs 140 gr silhouette mold that shot under 2 moa in my factory ruger 280 rem. barrel ... email me if you would like to borrow it ... heck work up some good future loads for my 7-08 barrel ( g ) .   i like the 7mm for cast ... low recoil and not everybody is shooting it.

ken

 

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Bud Hyett posted this 14 January 2017

I have had experience with both the .7mm-08 and the 7mm BR in rifles plus the 7-30 Watters in a TC Contender. I like the 7mm bore. The current 7mm BR is a 788 action with a heavy barrel in a Bishop wooden benchrest stock. It is the height of a late 1960's benchrest setup. The 7mm-08 is a Remington 700 Varminter 

FACTORS:

  • You lose one half of the difference in bullet diameter for touching the next ring, you must gain .014 iaccuracy
  • You only gain ballistic coefficient when shooting the 168 grain bullets, 
  • The 7mm-08 case will perform well, the 7mm BR case is a little under-bore for maximum cast velocity
  • Twist rate is seldomly mentioned in these discussions, it is a significant factor 

My experience is that the throat for the long 7mm bullets is critical. The RCBS molds have thick bodies and push back into the case with the gascheck below the neck,Even with this condition, at higher velocities the bullet showed the best tendency and desire to group. 

You will need a throating reamer and patience to get the throat long enough ot gently seat the bullet into the leade. This is for the rifles and the Contender, the leade was too short. The SAECO #073 bullet engraved. The bullet was linotype and very hard, but I did push the velocity above 1800 fps.

Personally, I would experiment with the 7mm-08 leade and the load before cutting the barrel back. The 7mm-08 I bought was used and cleaning took a while to get the copper fouling out before It began to repeat with cast bullets. Then it wanted to shoot, I was simply working too many hours to give both rifles the needed attention to get the most of each caliber. I think the next step will be to buy a 1 1/2 degree Included throating reamer and take the time ot make the throat lightly accept the body of the bullet.

Molds:

  • RCBS 7mm-168-FN
  • RCBS 7mm-168-SP
  • RCBS 7mm-145 SIL
  • SAECO #073
  • SAECO #071 experimental
  • SAECO #072 

Now that I am retired, I plan on regaining testing and possibly competitive experience with both rifles. 

Farm boy from Illinois, living in the magical Pacific Northwest

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frnkeore posted this 15 January 2017

I'm not sure why you are thinking recoil is a issue. A 160 gr bullet at 1500, in a 13.5 lb rifle, shouldn't have much recoil, at all.

Frank

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45 2.1 posted this 15 January 2017

Decrease the hardness of your bullet to less than 10 BHN and try again. At the low velocity you're shooting any fit problems show up as bigger groups (and most bullets are undersize to begin with). Also at the lower pressures you're shooting, neck fit is important. I've tried neck clearance from 0.0007” to 0.003” and find tighter is better until you get the pressure up to 40K or more.

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joeb33050 posted this 15 January 2017

IN YOUR OPINIONS, I am currently considering converting my 7-08 to 7mmBR in a quest to lighten my recoil but hopefully tighten up my groups. What I would like is y'alls input on how the 7mmBR performs, pitfalls or problems with the cartridge. I have been seeing a LOT of winning scores from the 30BR folks. But since I already have a 7mm bore, I was going to try this rout first. All comments welcome. Currently I'm using WW/2% Tin water dropped. A Lyman 7mm 160g #287641 mould. Aluminum gas checks, glued. Lee Alox cut 20% with mineral spirits. Average velocity, 1500fps. My groups generally hover between 3/4-1” overall. Thanks y'all.

If your gun shoots 5 shot 100 yard cast bullet groups averaging between 3/4” and 1"; unless your gun is a HEAVY or UNRESTRICTED class kind of gun; then changing your barrel to 7BR will probably not reduce group size. And if the change reduces group size, you'll need to shoot MANY groups with each chambering to prove it.

Changing from a larger to smaller cartridge case, shooting the same bullet, will reduce recoil only a tiny bit; only the powder charge weight difference/reduction will change the recoil. You'll never notice the difference in recoil.

Changing from a larger to a smaller cartridge case, 7-08 to 7BR or 308 to 30BR or 32-40 to 32 Miller or ... is alleged to increase accuracy because of several largely imaginary benefits. The smaller-shoots-better theory is unsupported by my 223 vs. 22-250 experience and the ASSRA 32-40 vs the-dozen-smaller-case experience. There's no data.

John Ardito chambered barrels at each end, different chambers, sometimes with an action at each end. This is the best way I've ever heard of of testing the smaller-shoots-better theory. Where are the TFS articles by John that show his results?

joe b.

   

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David Reiss posted this 15 January 2017

My wife expressed interest in deer hunting a couple years ago, so a little over a year ago I got her a Mossberg Patriot walnut stocked 7mm-08. She is not a very big person, but handles the recoil just fine. I started her out on CB loaded and have since fired nothing but CBs in it. I had expected groups around 1.5” or slightly larger, but was pleasantly surprised when my groups were just under 1” using a NOE 287-150-FN (WW w/ 2% tin added water quenched) lubed with LBT blue and 24 gr. of R-7. I have 3 more 7MM molds I plan on trying this spring & summer. But those groups said to me that this sporter has more promise, but maybe not. However groups under MOA are good enough for me. I would try and work with any rifle showing sub MOA for a long time before considering rebarreling.

I agree also with those that have said the recoil difference would be negligible. Since my wife can handle a 150 gr CB., I think she will like loads with 130 gr CB. I just can't see that the load you are using is producing much more recoil.  

David Reiss - NRA Life Member & PSC Range Member Retired Police Firearms Instructor/Armorer
-Services: Wars Fought, Uprisings Quelled, Bars Emptied, Revolutions Started, Tigers Tamed, Assassinations Plotted, Women Seduced, Governments Run, Gun Appraisals, Lost Treasure Found.
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Brodie posted this 15 January 2017

Recoil is a function of momentum; or : P=MV (momentum = Mass X Velocity).  Since Mr. Newton and his laws rule (especially the second one) M1V1=M2V2.  Or, what goes out the barrel comes back through the stock into your shoulder.  Therefore, as long as the mass and velocity of the bullet remain the same the Recoil will remain the same irregardless of the size of the case.  You might get a little more accuracy because the ignition might improve in the smaller case, but you will have the same recoil.  Brodie

B.E.Brickey

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