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  #1  
Old 04-12-2012, 03:00 AM
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Query regarding the overall efficiency of a 'down loaded' case


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Whilst impatiently waiting for the accuracy of my ’78 vintage .243 Browning B78 High Wall to noticeably deteriorate I’d very much appreciate some technical advice relating to it being ultimately upgraded with a 6.5mm caliber 28” heavy varmint barrel.

For consideration – bearing in mind that the alleged throat & barrel burning 6.5x284 cartridge has a comparatively greater powder capacity than the 6.5x55 SE (eg. 68 Vs 58 Grains of water), would the cartridge’s overall powder combustion and associated pressure efficiency be dramatically effected if it was loaded DOWN to fractionally more than the 6.5x55’s potential performance in a STRONG falling block action rifle like the B78 (eg. when predominantly hunting feral cats, foxes and ‘tagged’ kangaroos with Hornady 95 Gr V-Max’s out to about 300m) ?

Note, I have seriously contemplated the notion of minimizing the likely stretching of the standard ’55 SE cases (arising from my intention to shoot near max loads) by alternatively opting for the Ackley Improved version. However, the CUSTOM Redding dies are quite expensive plus there’s the ‘trouble’ of initially fire-forming the standard cases. Thus my 6.5x284 ‘down loading’ query, with the peace of mind of having the bonus option of loading it up for near maximum performance (eg. when infrequently hunting wild dogs, feral goats and pigs using Hornady 129 & 140 Gr SST’s out to about 400m).
Cheers, Ross.
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Old 04-12-2012, 11:16 AM
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In general, the lower the peak pressure the less efficiently powder burns. When you go to a larger chamber volume you have a smaller expansion ratio, and because the gas expands less, for a given peak pressure, the pressure drops less from by the time the bullet gets to the muzzle, so velocity is higher.

Given the above, the bigger chamber will have to be loaded to a lower peak pressure to hold velocity down to a smaller cartridge's performance level. But because of the burn inefficiency that will result, it will actually require more of any given powder than was used to get to the same velocity from a smaller chamber. It will also tend to fill the case poorly and have less regular ignition.

All the above having been said, Geoffry Kolbe claims throat erosion is strongly associated with peak pressure. Temperature and total heat content of the gas affect it as well. So it seems to me you could load the 6.5-284 to, say, 20% lower peak pressure, say 49,000 psi, have maybe 100 fps more velocity than the Swede and still get less throat wear than the 6.5-284 normally has associated with it. You will still be using maybe 10% more powder per round than the Swede, so it will about that much less efficient using the same powder, and you will likely have poor case fill and may need to use a magnum primer for that reason to get best velocity consistency. But none of those things means the group sizes will necessarily be bad.
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Last edited by unclenick; 04-12-2012 at 11:18 AM.
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  #3  
Old 04-12-2012, 06:27 PM
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Hey there, Ross...long time, no type!

I've never been particularly enamored of rebated rim cases and would be even less likely to choose one for a standing breech firearm, where that case head and rim are not fully surrounded. Now, I'm not a gunsmith and I don't know that it matters a whit, but I prefer rimmed or rimless designs for break-action or falling block action firearms.

With that being said, I think your concerns are well-founded, but your conclusions pertaining to the effectiveness of full-house 6.5x55 (and AI) loads are selling the old Swede a bit short! To put it succinctly, the standard or AI version will be more than sufficient to meet the goals mentioned above without the need for a rebated rim cartridge.

In fact, if you want more power than a 6.5x55AI, I would suggest two other options: The 6.5-'06 (256 Newton?) or, if you really want a rebated rim, fire-breather of a cartridge...the 6.5 Leopard! If you go with the Leopard you won't have to suffer interminably for throat erosion to necessitate a new barrel, or setting one back, at the very least.
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  #4  
Old 04-12-2012, 11:15 PM
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Nick, reading between the lines it appears that ‘down loading’ the 6.5x284 basically depreciates its overall potential burn efficiency – thus adversely effecting velocity and pressure. A possible remedy might be to use a coarser / slower burning stick powder which would fill the case more, hopefully resulting in more regular ignition and thereafter, combustion in the proposed 28” barrel. However, considering the comparative amount of powder required for the 284’s considerably larger case volume my absolute gut feeling repeatedly rests with the alleged view that the Ackley Improved 6.5x55 SE offers the optimum 6.5mm performance, combustion efficiency, and negligible ongoing case maintenance. Sure, the related Custom Redding dies are expensive but the B78’s my only centrefire rifle. In short, I’m not likely to be truly happy with anything else. Sincere thanks for clarifying the matter – your wisdom, as usual, is very much appreciated.

Jason, I’m inclined to agree with you regarding the 6.5x284’s rebated rim case being not overly suited to a falling block design – as opposed to being ‘safely encased’ in a bolt action. As with Nick, I greatly appreciate your sound advice – except for re-chambering my cherished B78 to a 6.5 Leopard !! Talk about being a potential throat burner…..no thanks. I’m really hanging out for this much talked about but rather costly 6.5mm upgrade. In the meantime, I’m hoping to further diminish the B78’s accuracy over the next few days hunting wild dogs up on a sheep farmer’s 10,000 acre property. How I love time camping out in the bush.

Cheers, Ross.
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  #5  
Old 04-13-2012, 04:37 AM
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Ross,

A good friend of mine here in Indiana is having a custom rifle built to commemorate a very personal event in his life. It is uncommonly beautiful, with a stock made from a wood so finely figured as to verify the existence of a higher power. This once-in-a-lifetime firearm could be chambered in anything my buddy wanted and he chose...6.5x55 Swedish.

When it's done, and with his permission, I'll email you a picture of the gun. It's nothing short of enduring artwork.
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  #6  
Old 04-13-2012, 10:28 AM
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Ross, I wouldn't sweat the "efficiency" or whatever. If you have published loads, then use them. Who cares how efficient it is as long as it does the job? There is a high likelyhood that you can find a modest load that will deliver the accuracy you want, within published data.

And yes - stick powders are a little more forgiving of air space than ball powders.
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  #7  
Old 04-13-2012, 10:51 AM
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Have you considered the .260 Remington or a 6.5 Creedmoor?
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  #8  
Old 04-14-2012, 11:45 AM
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Another option would be to have your existing barrel rebored to .264 and have a finish neck reamer run into the chamber and you would have a .260 Remington. Great cartridge, cheap dies, and you could just neck up your .243 Win brass and be ready to load.
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  #9  
Old 04-16-2012, 03:23 AM
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Jason, I'd greatly appreciate a pic of your friend's custom built 6.5x55 rifle, in due course. It certainly says something about the historic and proficient cartridge.

Kludge, yes I've seriously considered both the .260 Rem and the 6.5 Creedmore. However, I'm essentially after a bit more potential performance, but without the renowned wear & tear associated with the likes of the 'hot rod' 6.5x284.

Black Mamba, I've previously made some inquiries about reboring my existing barrel to a .264 but, from memory, I was advised that a new barrel was the overall better performance proposition - particularly in light of the fact that a 28" barrel would be more conducive for the efficient combustion of slow burning powders in the slightly larger 6.5x55 SE case (and even more so with the Ackley Improved version).

MikeG, considering the fact that the 6.5x55 SE has been around since Adam wore diapers it must certainly be deemed as an 'efficient' cartridge. Accordingly, the less tapered Ackley Improved version, from a reloader's perspective, would simply make it an even more effective round, particularly in relation to case maintenance. For better or worse, the likes of Unclenick have drummed into me over recent years some of the finer points about 'efficient' reloading techniques. For example, using the short necked .260 Rem case in a long throated chamber (favoured as I also intend to load the longer 140 Gr bullets for culling larger game) would result in the shortish length Hornady 95 Gr V-Max bullet (my intended primary hunting projectile) having both minimal neck bearing contact and an 'inefficient' considerable jump to the lands. In contrast, the long necked 6.5x55 SE case's more aptly suited, and thus more 'efficient' for the intended purpose of endeavouring to extract the optimum performance from the cartridge. In fact, I've encountered numerous people, on various similar forums, wishing that the over-bore 6.5x284 had a longer neck. In short, the '55 case appears to be the pick of the 6.5mm caliber crop.
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Last edited by Ross Clifton; 04-16-2012 at 03:34 AM.
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  #10  
Old 04-16-2012, 05:52 AM
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Well, I used to believe the gun magazine articles, too. Stuff like short necks being a big problem, and inaccuracy from too long of a jump to the lands. It all made sense till I got a rifle or two that had the "problem" and, yet, still were accurate and quite useful in the hunting fields. The examples I could give are just that - examples - and not important to the discussion except to note that they exist.

So... again, get what you want, and don't let someone else define "efficiency" to the extent you feel bad about what you want. Yes, there is a mechanical efficiency in how much powder burned vs. how much bullet goes down the tube. But, the .22 short is the most efficient of all, and so what? Sometimes, to make the end result happen, we have to burn more powder so that's what we do!

I know my .257 Weatherby, long throat and all, is not as efficient as my .257 Roberts but I shoot it anyway and like them both.

Good luck.
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  #11  
Old 04-17-2012, 07:55 AM
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I suppose efficiency needs a definition, and more than one is used. For interior ballistics its the percent of potential energy stored in the powder that's converted to kinetic energy in the bullet. For exterior ballistics it boils down to ballistic coefficient in combination with optimal spin rate to minimize velocity decay. For terminal ballistics, in hunting it's usually the least destructive load that still achieves DRT, as that introduces the least meat damage needed to avoid blood trail tracking, which improves efficiency of the hunter's use of his digestive tract. The military probably looks at efficiency as the round you can logistically carry most of into theater that will still stop enemy soldiers effectively and wears barrels out most slowly.

I usually use the first definition by default, but others are valid in their context.
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Last edited by unclenick; 12-23-2012 at 08:12 AM. Reason: typo fix
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  #12  
Old 04-28-2012, 05:52 AM
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Interesting discussion. Brings back a lot of memories of the days when I was fiddling with wildcats.

I was prompted to dig out the manuals and look at the loads for the 6.5 Swede and the 6.5-.284. There's only about 100 fps difference between top loads given roughly the same case length, and maybe 6 gr. more powder. The biggest difference, obviously, is the sharper shoulder angle of the 6.5-.284. That's the part that contributes to the throat erosion in my opinion.

The Ackley version of the Swede is certainly a good option, but if I was going to build a 6.5, I would opt for the 6.5-06. That case design is one of the most efficient there is over a very wide range of calibers, and also one of the easiest to load for. I'm a 7mm fan, so one of my very favorites is the .280 Remington. It is very tractable in terms of bullet weights and load levels. Of course, the same can be said for all the cartridges based on the 06 case, from 25-06 all the way up to the .375 and .416 versions.

That would also be an easy upgrade for the Browning. Set the barrel back one thread and rechamber for the 6.5-06. That would be superb for long range antelope and deer, sheep if you can get close enough, maybe even elk and easy on throat.

Just some random thoughts...
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Old 04-28-2012, 06:50 AM
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Interesting results. I show more like 200 fps difference with heavy bullets and 300 fps difference with light bullets. But that's staying within published pressure limits. The 6.5×55 can actually be loaded to the same pressures as .308 can in many rifles, and that reduces the difference (though it also increases throat wear).
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  #14  
Old 05-10-2012, 02:26 AM
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Rifter, you suggested that the sharper shoulder angle of the 6.5x284 contributes to the throat erosion (namely, 35 Vs the 6.5x55 SE's 25 degrees) and then stated that the Ackley version of the Swede is certainly a good option. The latter's 40 degrees - inferring that the '55 AI would result in even more throat erosion !? For consideration, I stand to be corrected but the 6.5x284's poor throat record appears to stem from the fact that the case's powder capacity is over-bore for the caliber (as per the 6.5x06 case) and that it has a comparatively short neck (compared to both the 6.5x55 SE & AI and 6.5x06) - thus likely to direct more of the burning powder into the .284's throat, as opposed to the '55 AI case's neck.
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Old 05-10-2012, 04:57 AM
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Ross,

Neck length and shoulder angle may play some small part in throat erosion, but the preponderance of such is caused by large volumes of super-heated gas in a smallish bore. The 264WM and 6.5 Leopard jump to mind. The 6.5x55 AI, at 62 grains of capacity, has the least room for powder of all the rounds mentioned, yet still delivers excellent performance from a century-old case design. I don't think you could go wrong with the Improved Swede, for the goals you've stated.
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Old 05-10-2012, 08:16 AM
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Quote:
Originally Posted by Ross Clifton View Post
Rifter, you suggested that the sharper shoulder angle of the 6.5x284 contributes to the throat erosion (namely, 35 Vs the 6.5x55 SE's 25 degrees) and then stated that the Ackley version of the Swede is certainly a good option. The latter's 40 degrees - inferring that the '55 AI would result in even more throat erosion !? For consideration, I stand to be corrected but the 6.5x284's poor throat record appears to stem from the fact that the case's powder capacity is over-bore for the caliber (as per the 6.5x06 case) and that it has a comparatively short neck (compared to both the 6.5x55 SE & AI and 6.5x06) - thus likely to direct more of the burning powder into the .284's throat, as opposed to the '55 AI case's neck.
Shoulda been more specific. The combination of the fat case body and the sharp shoulder funneling into a small bore combine to hold the hot gas for just a tick longer so that the plasma that exits the case is just like a blow torch being fed by raw oxygen. The Swede has the narrower body so the burning powder starts moving down the barrel sooner, even with the Ackley shoulder.

It also doesn't help that some of these fat overbore cartridges get loaded right up to and often past the redline pressure-wise. That only aggravates the problem. Most of the Ackley versions add a couple grains of powder but the pressure stays in the same vicinity, and those that gain the most benefit are often slightly underbore. Leastways, that has been my experience.
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Old 05-10-2012, 11:59 PM
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Rifter,reading between the lines, the Swede's narrower body appears to offer a potentially slower combustion rate compared to a fat case body (logically due to the powder having less direct surface area exposure to the primer) thus supporting your view that the burning powder starts moving down the barrel sooner rather than 'scorching' the throat - as evident with such over-bore large powder capacity cases like the 6.5x284.

Jason, in turn, the Improved '55 SE definitely presents itself as being the most ideal caliber and cartridge upgrade for my old B78.
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Old 12-01-2012, 11:42 AM
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At normal barrel lengths a few inches of extra barrel length is not going to increase the 'combustion' of the powder. The powder that is going to burn will have burned long before it gets anywhere near the muzzle. Powder ceases to burn because the pressure has dropped too low. 'Slow' powders require high pressures to burn efficiantl while 'fast' powders will burn at low pressures.

An example from QL

As an example 6x55 48 gr RL19 140 gr bullet. @ 56,816 psi 97.8% load density 24 inch barrel 2825 fps 28 inch barrel 2928. A gain of 103 fps for 4 inches of barrel. Muzzle pressure 24" = 11,394 psi, 28" = 9507 psi. There is not enough powder being burned that last 4 inches to maintain the pressure. The extra velocity is due to the extra 4 inches f expansion space.In 24 inches 98.89% of powder burned. In 28 inch 99.47 % burned. The 0.58% is so little and has so little time to exert force on the bullet and that at a time when the pressure has already dropped to half that of a .22 rimfire that it isn't worth considering. At any significant range it won't be noticeable.
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Old 12-21-2012, 07:32 PM
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When loading down it often pays to change powders. Try going to a bulkier powder and/or faster powder. Loading density is very important to consistency. A faster powder will help with complete combustion and efficiency.
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