Jump to content

Question for the F/TR guru's (Twist rate)


Elwood

Recommended Posts

A friend of mine is having his 308 F/TR rifle rebarreled and he's asked me a question on twist rates that I can't answer.

 

His dilemma is does he choose a 1-11 twist and shoot the 155's or will a 1-10 enable him to shoot 155's up to the 185 grain bullets? Is anyone having success with the 185's?

 

He also mentioned shooting the 210's but I'm guessing the chamber would only allow for the 210;s and nothing else? if he went with the 210's which twist?

 

 

Link to comment
Share on other sites

Hi Ian,

 

The 1 in 11 will do the 155's and the 185's, my best results have been with the 185's in an 11 twist barrel and that includes the 1000 yard GB F Class record. You can set the throat to do both nicely but if you go over the 200 gr mark then it will be pretty much be dedicated to the heavies.

 

Cheers

 

Stuart

 

PS if he wants to use 210's then he will need a 1 in 10"

Link to comment
Share on other sites

I've had very good results with 155-190 with a 1:12 and 185-230 in a 1:10.

I'd say if he has any desire to go heavier than 185s, he should go with a 1:10.

In my experience the 185 Bergers shoot very well from a 1:10 even with a big jump, so even if he had it throated for heavier bullets it will still give him options.

All IMO, of course...

Link to comment
Share on other sites

Hi Ian,

 

I have run both 155 and 185 bullets through my Sako TRG-22, which has a 11 twist barrel, with success and even 190 grn SMKs. I tried pushing them fast to get the 1000 yard distance but it didn't like that, but keeping the Berger 185s to 2550-2600 fps gave me the accuracy I expected.

I'm now using a 10 twist Benchmark barrel on another rifle and I'm more than pleased with the accuracy and speed it is giving with 155 and 155.5 bullets. I went for this twist because I wanted the option to re-throat for 200 - 210 grn bullets and some point in the future. Whether I actually do this remains to be seen.

HTH

Link to comment
Share on other sites

Ian, a lot of people are simply sticking to either 1 in 10 or 1 in 11. Today's bullets are so well made and balanced dynamically that serious theoretical over-fast rotation doesn't affect their long-range performance. Many of the top league FTR boys use 10s and have chambers throats such that they can opt for 155s or 210s effectively on the day. The main trick is to use the 155.5gn Berger BT Fullbore + the 210gn Berger BT (NOT VLD). The 155.5s are amazingly jump tolerant bullets and the 210 BT doesn't need an incredibly long suit-one-bullet-only throat. (It's also much easier to tune than the 210 VLD and is cheaper / easier to find than Hybrids.)

 

FWIW, the 185gn Berger LR BT 'Juggernaut' works fine in a 1-13" twist in all but the coldest conditions and Bryan Litz considers 12" optimal for it. So, if your friend has this bullet as his 'upper limit', this twist rate is ideal. It also matches the new generation of long 155s to a Tee.The 185gn Hybrid is a tenth of an inch longer than the Juggernaut and an 11" twist is optimal for it, but not many people on the GB circuit are using this model. In fact, the Hybrids have not picked up the following in FTR that they have in 7mm, and where used, the outstanding models are the 168gn and 200gn examples. The former works brilliantly in 12-inch twist, the latter needs 11" and won't be upset by 10".

 

Not very many top GB FTR shooters use 185s tending to jump from 155s to 200/210s. The 185 Juggernaut is an outstanding bullet and was used (compulsorily) by all US and Canadian FTR team members at the FCWC at Raton last year, and I would think by three-quarters or more other US and Canadian shooters. The view here is that if you're going to live with the (noticeably) heavier recoil and torque of a 'heavy', you might as well go a bit further and adopt either the 200gn Hybrid or 210gn LR BT. There is a problem with the Juggernaut too that you'll recognise as a 7mm F/O competitor - as one of Berger's very best selling models worldwide, most of the time you can't get hold of the things, and no dealer is ever going to give you a discount even if you order 5,000.

Link to comment
Share on other sites

I'd agree with Laurie on all points there. Availability was one of the reasons I stuck to the 1:10 most of the last two years. It will shoot almost anything well so if I can't get hold of the 185s, I just pick whatever I can get - 200, 215 or even 155 if I'm really desperate!

 

My 1:10 Bartlein is throated to accommodate 230 Hybrids. Just for a laugh, we put some NRA issued RUAG 155gn through it at a 1000yd practice session a few months ago and I'm not exaggerating, the group was as good as I was getting out of the 185 JTs or 200Hybrids I was using at the time. Now, the conditions may have been favourable for the 155s that particular day, but it just goes to show.

 

The really interesting thing was that those 155s were having to jump 1/2 inch to get to the rifling.

Link to comment
Share on other sites

Laurie's and JSC's posts are particularly of interest to me because I have been informed that if I throat my 10 twist barrel for the 200/210 BT's it will no longer be good for the 155 Berger Hybrids or 155.5 Berger Full-Bore, or any 155 bullet for that matter.

Is this correct or are you saying that the Berger 210 BT will seat and shoot OK with the current throat? Or, do I understand that it should be throated for the 210 and give the 155s a big jump?

I'm currently using COAL is 2.940" which gives a 0.020" jump.

Link to comment
Share on other sites

The 185 and 210 LRBTs are relatively short bullets for the weight at least so far as the point where the shoulder / shank junction starts to contact the lands. I believe the 185 and 200 Hybrids need more freebore, but you'd have to get a user to confirm or deny that.

 

Since I never got into anything much above 185/190 level, I can't comment on how the 155.5 / 210 BT users set their chambers up. What I do know is that The Gun Pimp, Stuart Anselm and self kicked off in FTR years back with the 155gn Scenar which was THE bullet back in them days and Vince got a PT&G reamer based on this bullet seated optimally in a Lapua case. It turned out when the 185gn Berger BT later appeared, this chamber was also spot on for it, and as I was using a 1-10" barrel (not through particular choice, rather it being available) simply worked up new loads for the heavier bullet and got them up to 2,825 fps which is the ideal speed for this model. (But not for barrel life it turned out as this was obtained with Viht N550.) I also used the 210 BT fairly successfully in this chamber in short-range tests but not in matches, although it was bit short for the bullet - but I didn't like it anyway, and got better results with the 185.

 

The 155.5 is an amazingly jump-tolerant bullet, and is almost certainly the one to stick to in this weight category especially if going in for compromise freebore arrangements. The Hybrid is NOT jump tolerant despite what Bryan Litz tells us. It appears to only work with very large jumps, 0.030" minimum, and I hear of 40 to 50 thou' being used. I'll have to sort mine as I bought 1,000 and am running out of 155.5s - I did finally get a large primer Lapua case load that worked recently, so need to do same for small primer Palma brass. The 168gn Hybrid which I consider an outstanding performer in two very different 10 and 12 inch barrel rifles only shoots well fopr me if treated like a VLD and seated 'in' ... but I've not tried it with the very large jumps that some users find the Hybrids need. (This also appears to apply to the new model 155gn Sierra Palma MK.)

Link to comment
Share on other sites

I don't think there's a definitive answer to that, because it depends on a few things, such as what velocity you need to make (dictating how much powder you need to get in the case and hence the minimum OAL you need) and how jump will affect each bullet type.

 

I didn't start out thinking a long throat would work with lighter bullets, but in my experience it does with the bullets/loads I've been using and a bit of experimentation, so I ended up keeping my 1:10 long throated barrel on the gun most of the time and not worrying about the jump. Sometimes I've shot 185s, 200s and 215s through it in the same meeting at short and long ranges and the results have been very good.

 

Going by my own experimentation and testing and then comparing it with what I've read and heard, I've reached the conclusion that a lot of people seem to be worried about jumping bullets more than 40 thou' and so don't even try it.

 

If you want to use one specific bullet all the time, then it makes sense to use a barrel which bests suits it and maybe a 1:11 or 1:12 would be the best choice for the 155s, but a 1:10 makes a very good all-rounder if you're going to shoot a variety of bullets and aren't too hung up about jumping some of them a long way, such as 500 thou' in my case :)

 

It's also worth remembering that the Juggernaut and Tactical OTM (which I think is very similar, if not the same design) were developed to be very tolerant to jump so they would work in magazine fed and service rifles and also to cope with the transonic zone well without getting too unstable. I've shot the 185JTs at 1200 yds knowing they would be subsonic well before the target and had some amazing results out of them.

Link to comment
Share on other sites

My 1:10 Bartlein is throated to accommodate 230 Hybrids. Just for a laugh, we put some NRA issued RUAG 155gn through it at a 1000yd practice session a few months ago and I'm not exaggerating, the group was as good as I was getting out of the 185 JTs or 200Hybrids I was using at the time. Now, the conditions may have been favourable for the 155s that particular day, but it just goes to show.

 

The really interesting thing was that those 155s were having to jump 1/2 inch to get to the rifling.

 

 

Jason, I'm not too surprised. They seem to shoot well in everything. I suspect if you'd had a chronograph you'd have got a shock though. I ran some through a 22-inch barrel Howa Varminter with a fairly long throat and they failed to hit 2,600 fps. The load has been optimised for max pressures in a 30-32" 'tight' TR rifle barrel set up for Sierra 155s. Simply changing to a standard SAAMI barrel as we all use in FTR (.300" bore / 0.3080" groove dia.) will likely see a large MV reduction irrespective of throating freebore. (The TR guys and girls still use blanks with 0.298” bore and 0.3065 to 0.3075” groove diameters.)

 

Bearing in mind that the NRA / RWS cartridge produces 2,925-2,950 fps from a 30-inch tight barrel in good condition, that barely keeps the old Sierra 155 supersonic at 1,000. So, heaven knows what the terminal velocity was in your long throat chamber and standard internal dimensions. The bullet is remarkably stable at trans and subsonic velocities - providing conditions are stable. US Army long-range trials of the old US 173gn FMJBT .30-06 and early 7.62mm sniper / match bullet found that 1,225 fps (ie 100 fps above the speed of sound) was the critical terminal speed. Drop below it and the bullet would still perform well in calm conditions, but air and wind changes had a much larger effect than they should have from simple BC / speed calculations.

Link to comment
Share on other sites

We were putting them over a chrono and they were up at 2850 or so from memory, so not much of an MV drop as a result of the big jump.

 

I've dug out the plot sheet and 10 shots gave a 1/2 MOA vertical spread at 1000yds out of my 1:10 Bartlein!

 

I almost bought some to use for matches...

Link to comment
Share on other sites

Thanks all, this is great info, but the advice I'm going to pass onto him is to shoot F open from now on as it would appear to be less hassle. One 7mm barrel with a 1-9 twist and a 180 grain bullet, sorted. :)

 

 

Ah ... but FTR people have to get it right first time. Their barrels last a long time, while you guys are able to change your spec every few ..... ? :) (Anyway, Berger has resumed work on the 195gn Hybrid - won't you all be changing everything for 2015?)

Link to comment
Share on other sites

  • 2 years later...

Just resurrecting this thread. My chamber is 16 thou off the seating point of a 155.5 so even with a minimal seating depth the jump is going to be pretty large. Would a 100 thou jump and 84 thou seating depth be out of the question with this bullet

Link to comment
Share on other sites

I use a .298 x .3075, 30 inch TR Krieger, my experience with these specifications and the Berger 155.5 FB means I seat close to the lands, ie 5 to 10 'thou off. Any more that 10 'thou and the groups open up. During testing, seating out at up to 50 'thou off never worked in this particular barrel, contrary to the general view. Barrels do perform differently of course.

Martin

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use and Privacy Policy