6mmBR Barrel length?

[LukeSoutherner]

I have had a look through the forums here, but can't find any postings that answer my question specifically (I could, however, be wrong!).

 

I am having a barrel made for me AI AE mk3. It is currently chambered in .308.

 

I've decided on the 6mmbr as it is conservative with powder usage, and my Police force will allow it for fox as well as Roe. I also love the idea of lower recoil, and it being easier to moderate. I live in a valley, and sound travels a long way - the neighbours have complained before! So, the quieter, the better. And, before anyone says it, I know that I cannot silence the flight of a supersonic bullet, only the gasses at the muzzle.

 

Now, I like small rifles (no jokes there please!), and so I'd like to go with the shortest barrel I can get away with. But, I need to get the velocities (2400fps) and power (1000ft/lb) to shoot roe in Scotland. I wold also like to be able to shoot it out to 500 yards. As you can see, I'd like a rifle that I can almost do anything with!

 

What is the shortest barrel length could I get away with do you think?

 

I have read through a lot of the info on 6mmbr.com, but many of the barrels are at 26"+ length. Could get away with an 18" barrel? Or would it be more sensible to go with 22" - which in my opinion is very long!

 

Thanks for your help,

 

Luke

[Laurie]

Luke,

 

it'll help people answer your query if you mention your barrel's twist rate, the bullets you plan to use for the three purposes (three assuming that 500 yards is paper not live quarry), and how the chamber is to be throated as the resulting bullet used + COAL affect charge weights and pressures materially in such a small capacity cartridge.

[LukeSoutherner]

Hi Laurie,

 

Having never had a barrel made before, I'm not completely sure the answer to all your questions. Having spoken to Stuart at Osprey, I'm going to settle on a 1:8" twist. I want to shoot bullets up to 100gr, but I am not completely sure which bullets yet. Maybe I'm going about this all wrong. I learnt what my .308 barrel liked to shoot through trial and error for both which powder weight, bullet weight, type and COAL. How would I go about working out the answers to your questions? I assumed I'd have to work this out afterwards, but it sounds like I could go from the other end, so to speak, and work out exactly what I want to shoot through the barrel, and have it made to that spec.

 

Cheers,

 

Luke

[gbal]

Luke,asking -on here eg- is sensible (you could have saved a lot by taking shared experience with 308-there are some sound options,as it is very popular.Whatever)

This is less so with a short 6BR-very very few will be short barrels,as the main reason for a 6BR is accurate performance at distance from a very shootable cartridge.It is nonetheless a very competent fox and small deer cartridge.You will lose quite a bit of it's longer range ability,of course,but should be ok to 500 for target use.

Laurie is right in detail,but I can see your general point too-which specific make/design of bullet can wait-we can progress a bit though,within what is an unusual compromise proposed package.Thus:

 

The 6BR is not much behind the standard 243 (varies with bullet weight etc,but bear with me-at least until 6BR data come in (if they ever do for short barrel). You give 2400fps and 1000ft lb as your minima:well,that is what a 24" 243 85g SGK @3300mv is giving at 300y (6/8 drop/drift ").

 

Again it varies,with velocity and just which inches are being removed (26 to 24 less velocity loss than 22 to 20 eg),but estimates/experience suggests maybe 30 fps per inch (very ball park) so you have quite a margin....

As for light rifle (can't see an AE as light,whatever its other merits), the Rem 7 had a 20" barrel,and the Ruger International an 18" barrel,both in 243 and around 7lb.I had a couple of rem 600s in 222 (scotland) and 243 with 18" barrels. Light and very manoeverable.

All these were easily beyond your minimum. You have to assume the 6BR will lose a bit though,compared to 243-but the margins look favourable for you.Get a mod-muzzle blast/flash can be considerable,esp in poor light!

(Actually for fox,small deer if there were any 'carry' /out of cab window work,I'd get a separate light rifle,something as above-it might lose a 1/2 moa absolute accuracy,but is more practical to 250y or so. I hope your 500y use is target only-energy is getting down quite a bit,closer to 500 ft lb,estimated,drop/drift ,drop drift 40/26 ,243).

 

You do have a wide choice of bullets-from the light 68 on up through 90,maybe 105.Both target and game designs-your choice of 1in 8 is good,bringing the heavier into play.Barrel shortening often has little real effect on accuracy-might even improve very light/whippy tubes....maybe!

 

OK-what you outline can be done,it seems-though it's unusual-why not,as a project.Of course,this has to be verified for 18" 6BR specifically....sorry I don't have that data-maybe someone will.

 

gbal

[Borisserge]

I run my 6mmbr with a 22" barrel, 1-8" twist and shoot 105gr with ease at approx 2750 ft/s without pushing the brass.

6mmbr have such little recoil that a 'heavy gun' is overkill on you to carry. Something more close to 8-9lbs fully scoped, moderated and with a bipod is very do-able, and still very accurate.

 

Regards

B.

[Borisserge]

Double post. ...

[gaz6br]

My 6br is 22 inch shoots the 105gr amax at 2750fps and 87gr vmax at 3100 with fantastic results and it's up for sale

 

 

105 amax 5 shot group

 

 

Gaz

[gbal]

Accurate Shooter site will give some (hot) loads with longer barrels,including the 105g and also lighter bullets...

 

 

 

 

 

 

 

 

 

 

 

 

 

gbal

[LukeSoutherner]

Thanks to you all for such informative and quick responses - my first time posting on a forum, having read UKV for a couple of year, and I'm grateful for your feedback.

 

Thanks for the offer Gaz, but I want to stick with what I've got already, in terms of the action - even if it weights a tonne! With the .308 barrel on it, it weighs 11b 3oz without any tackle on it - but it is a lovely bit of kit regardless. Maybe I'll have to work on my upper body strength instead of getting a shorter, lighter barrel...

 

Gbal and Laurie: the 500m that I mentioned was indeed for paper, not live quarry. I'd like a rifle that I can use on the range for target and fun, and then something that will perform on larger vermin and small deer. .243 is the obvious choice, but I think that West Yorks. Police can be funny about giving a variation for fox (overkill?!?). That's why, along with the superb accuracy, I was looking at 6mmbr.

 

Maybe 22" is the way to go, with the 1:8" twist. It might be the compromise between the 2 issues: size and power.

[Laurie]

The reason I asked about bullets you might use is that their length varies a great deal given the choice available for the 6BR (55 to 115gn, or more sensibly 55-105gn), and many people have the gunsmith chamber the barrel for a small range of bullets or even a single model. The issue is 'freebore' in that a chamber throated to suit a 105gn VLD seated at an optimal position in the case will produce a large jump by a short/light bullet. Doing the reverse, sees the heavier models have to be seated very deeply, so much so that it has a major impact on usable powder charges and velocities, potentially a key issue with your looking at very short barrels.

 

Most gunsmiths have a general purpose 6BR chamber reamer that provides a compromise amount of freebore, and that'll likely be what Stuart will use on your barrel. CIP / Lapua list the COAL as 2.340" for the 6mm BR Norma version of the cartridge (6mm BR Remington is a lot shorter), but that's actually quite a long freebore and sees the long 105gn Berger VLD seated pretty well optimally in the case. Most 6BRs I've seen come with a bit less, so suit a large number of lighter bullets.

 

Whilst the 100gn expanding bullets are ideally suited to the 243 Win, they are relatively on the heavy side for the little 6BR's internal ballistics. I'm not a deerstalker so have no inside knowledge on this issue, but I do have at least one Scottish stalking friend who is a fan of the 85gn Sierra HPBT deer bullet or 87gn Hornady equivalent (HP, not V-Max). I'd recommend joining 'The Stalking Directory' forum and asking about this amongst the large number of Scottish roe stalkers who are members on it.

 

https://www.sierrabullets.com/store/product.cfm/sn/1530/243-dia-6mm--85-gr-HPBT

 

Running the numbers through QuickLOAD, here is what it says for the 85gn Sierra GameKing at a likely COAL for the bullet and short barrels. Normally H. VarGet is the powder for this cartridge, but with a lighter bullet and very short barrels, the problem with all those in the 4895 / VarGet burning rate category is that they don't burn terribly efficiently - charge burns in the low 90% levels. Another problem that you get even with the little BR is VERY high blast pressures with short barrels. For accuracy, it's desirable to keep the muzzle pressure below 10,000 psi, but very short barrels raises it to the 15,000 psi mark. Since I'm not a moderator user, I've no idea what high blast levels do to their insides, but I don't imagine it's desirable.

 

Anyway, QuickLOAD suggests the good old fashioned and much underrated IMR-3031 is an excellent match for a short-barrel 6BR with 85s. Here what it says for the 85gn SGK at 2.200" COAL and a 30.5gn charge weight which is calculated to produce ~57,000 psi.

 

Barrel Length

 

14" ............................ 99.5% ................. 2,759 fps / 1,436 ft / lb .................. 15,133 psi

 

16" ............................ 99.9% ................. 2,859 fps / 1,543 ft / lb .................. 12,898 psi

 

18" ............................ 100% .................. 2,942 fps / 1,634 ft / lb .................. 11,162 psi

 

20" ............................ 100% .................. 3,040 fps / 1,714 ft / lb ................... 9,795 psi

 

 

The columns are left to right: charge burn % ........ MV / ME .................... Muzzle pressure

 

So you need 20 inches to get the muzzle pressure down below 10,000 psi. Nevertheless, 6BR is a very viable small deer cartridge even with very short barrels. The reason is its small case and hence low effective expansion ratio. Whilst 26 or 28 inch barrels are used in 6BR target rifles, even with 105-107gn bullets, the velocity gains are modest compared to 20" thanks to the small (28-32gn depending on COAL and bullet weight) charges.

 

I'll stress these are QuickLOAD results and while the program sometimes gives funny results, I wouldn't expect it to be wildly out in this case. There must be some 20-22 inch 6BR barrel users on the forum who can confirm these sorts of MVs, or otherwise as the case may be.

[gbal]

Luke,

Just a few quick points which occurred reading Laurie's detailed post:

 

Is not the minimum velocity 2450 fps (Scotland and England,all species)?Not a problem,though. 1750 ft lb also would bring Scottish reds in,with 100g bullets,and a useful margin generally for England roe.

 

You might want to choose a powder that is unlikely to be in short supply,so long as it performs (Vihtavuori is good stuff,Varget has been unobtainable...). 80 SGK class would be a good bullet ,better than the lightest options.

 

While open wrt 6BR and 243 (have/had both),you might phone your FLO and see his/her view,given your circumstances (they are both ,after all 6mm,and pretty closely matched);the FLO might agree to either-then the choice is yours,if more complicated!

 

I understand your liking the AE,fashions change: I felt in better shape for any shot if i'd been toting 8lb all in,for hours! :-)

g

[LukeSoutherner]

Thanks for inputting the data in Quickload Laurie, that helps A LOT. 20-22" seems to be a good compromise. I did not know about muzzle pressure having an effect on accuracy, so something new there. It is also interesting to see at what length the powder is actually completely burned, but that even when the powder is fully burned in an 18" barrel, 2" more gives another 98fps - a significant increase in my book.

 

I will speak to Stuart about the reamer he plans to use, and whether we should look at a specific set of bullets lengths/weights. For roe and fox, the 85gr sierra sounds suitable, and it would be good to be able to stretch out to those heavier head around 100gr.

 

The law for England & Wales is above .240" calibre and 1700ft/lbs for the larger deer species (i.e. not CWD or Muntjac).

For Scotland Roe= 50gr head, 2450fps and 1000ft/lbs, and the 3 larger deer species are 100gr head, 1750ft/lbs, and 2450fps.

 

Those numbers always confuse me!!!

 

I agree about Vhit. I chose that for my .308 specifically due to the lack of US powders available. Friends at the range moan about not having any Varget, while I keep on shooting with my N140!

[Laurie]

If you need the 1,700 ft/lb for English law, 22 or even 24-inches is better. Above 24, the extra MV from longer barrels falls off dramatically. 20 inches should easily meet your Scottish roe needs though if you limit it to that.

 

So far as 500 yard target shooting goes, depending on the freebore you end up with, you'll be able to use any of the 90/95gn match bullets around at the very least, maybe the 105s if the chamber allows. The 90gn Lapua Scenar at a modest 2,800 fps is still doing over 1,900 fps at 500 yards and sees almost identical wind drift per 1-mph of wind change to a good 308 Win 155gn bullet at 3,000 fps.

[LukeSoutherner]

Hi Laurie,

 

I think I met you without realising at Diggle on Thursday! I was testing finding Stuart's 6mmbr on the 100m range.

 

I was wondering if you could do me a favour and do another calculation via QuickLOAD please?! Having read the posts, talked to people at the range on Thursday, and looked at various bullets, I would like to have the barrel built around a 100gr round, and have found the Hornady 100gr Interlock, Sierra 100gr SBT, and Nosler Partition 100gr, which all look suitable for deer stalking. So, if you have the time, I'd be very grateful if you could try out the load data and barrel length as you did above for one of these bullets (probably the hornady 100gr Interlock, BC=0.405 (G1), and SC=0.242 according to their website). IMR-3031 seems to be a good powder to use, as you did before. I am getting more and more tempted to ignore my short barrel feelings, and simply go for a longer barrel 22"-26" so as to give myself versatility with the rifle, and so that it can be easily made red deer legal for Scotland. I think I want to know at what barrel length the round is large-deer legal with a 100gr head, and then I can make my final decision from there...I hope!

 

I am showing my naivety by saying this, but I am amazed by how much needs to be known before having a barrel made!

 

Many thanks Laurie,

 

Luke

 

P.S. At least I have the funds for the barrel now that I have sold one of my rifles!

[gbal]

Luke,while it might just be legal,6BR is a bit light for large Red deer-243 was always-and correctly-thought absolute minimum,and really only for the expert ,experienced shot,preferably for hind culling,certainly not a first choice main stalking cartridge. Margin for error-and they will occur-is just too small. There are far better cartridges for Red deer. 243 etc minima were as much a concession to existing rifles,than an optimum - which they are certainly not,and will sooner or later show their deficiencies.

There is a good reason why cartridges in the 6.5x55,270,308 class predominate for normal red deer shooting.Barrel length is no fix for a marginal cartridge,and length isn't an issue 'on the hill'-it can be a minor (convenience) one for woodland roe. All cartridges are a compromise for multiple use,but the choice has to be very good fit for purpose for the most demanding use proposed. 6BR isn't,here.

gbal

[gbal]

Luke, have a look back at the post "6BR Improved for deer,starting 9th Sept 2007.

 

Of about a dozen posts,none supported 6BR class for red deer.Despite it's excellent accuracy(ditto 6PPC),it just can't give these potentially well placed shots enough ftlbs.

 

Given appropriate bullet design,and equal shot (mis) placement,and there will not be 100% success,the argument boils down to :

 

For every failure using a 6mm class cartridge,a 6.5x 55/270/308 class cartridge would have at worst,no less chance of failure and often a better chance of success.

 

For every failure by the 6.5x55/270/308 class of cartridge,the 6mm class cartridges would have failed too,often even worse.

 

So,realistically and ethically:

A £600 308 class stalking rifle,sensibly chosen,would be my preference over any 6mm.If I could not have the '308' class rifle,I would not stalk red deer.1000 more fl lb is better than 1/2 moa,on paper.You only get one shot,anyhow.So where the second shot might have gone is largely irrelevant,but if you do need it,another 1000ftlb more,even a couple of inches away, beats anything through the same hole that failed the first time-you are in damage limitation anyhow-pretty unpleasant.

gbal

[LukeSoutherner]

Thanks for your opinion gbal. I will have a look at that thread.

 

I think my line of thinking comes from wanting the best of all worlds, but also a lack of experience. I can see what you mean in terms of stats, and that a slight miss on a 6mmbr means an unethical shot, while with a .308, it will have enough punch to kill the deer.

 

Would you feel that 66mmBR is adequate for Roe deer? Are we talking about diameter of expanded bullet as much as power? So a smaller deer will have a smaller kill cross-section, and hence a smaller diameter bullet will create, relatively, enough damage? While it would not with a .308.

 

Luke

[LukeSoutherner]

Having read the thread, it makes sense what you said. What irritates me is that the law is meant to guide people to make the right choices, and yet in this case what your experience says wrt to ethical kills appears more ethical than what the law says. Bureaucrats in Whitehall makes these laws, but they are not based on real life, nor ethics. No surprises there then.

 

I will think about what you said, and reconsider my choice.

 

Ta,

 

Luke