The UK's Finest Independent Forum for Shooters and Gun Owners. Est 2010.

I'm working up a new load for my 308 to use up a stock of the 155 grain 2155 Sierra MatchKing I have left.

I have a load for this bullet in this rifle that I slapped together without any real development work for an informal session on steel plates and have been meaning to replace for some time. It uses a fairly compressed charge of N140 in TZZ milsurp cases for about 2750fps in my 23" barrelled Sako 75 varmint.

It will shoot to about 1 1/2 MOA ( ten shot groups) out to around 500y, which is as far as I have shot steel plates with it.

This rifle is capable of shooting itty-bitty groups with mature hunting loads ( It really seems to like 125gr sierra SP over BL-C(2) for example, giving 3/8" groups at 100y and 165 Hornady interlocks over RL15 giving 1/2" at the same distance, five shot groups here.) and so I would like to better.

I'm going to try a few different powders in some sort of attempt at being systematic and also trying to work out for myself whether a particular barrel can like a particular powder across a range of bullets ( for example liking RL 15 and disliking N140 no matter what the bullet) or whether it is entirely individual load dependent.

To keep all the other variables the same I have made up batches of weight sorted RWS cases and would like to settle on a OAL to test across the powders.

The field expedient 155gr SMK load above was arbitrarily loaded to ~2.8" to fit in the mag but measuring the magazine I can go up to 2.900" and still fit in the magazine with a few thou to spare.

I have measured to the lands and it is around 2.880" or 2.300"( measured with a Hornady comparator thingy) to the ogive.

The thing is this gives bugger all shank in the case neck and the problem with this was demonstrated when the extracted case rolled off the desk and onto the carpeted floor, visibly knocking the bullet out of true.

I am now thinking I should seat the bullet as I would with my hunting ammo ( OAL starting point determined by having at least a calibre's worth of shank in the case neck) or whether to seat them out 10-20 thou from the lands as I understand is the norm in target shooting and just be careful with my ammo?

Opinions or criticisms of methodology are welcome.

No need to chase the lands unless you are certain you have exhausted the charge development one thing at a time

As above, the Sierras will be fine loaded to Saami Spec COAL, work through and fine tune your powder loads first.

phaedra1106 wrote:As above, the Sierras will be fine loaded to Saami Spec COAL, work through and fine tune your powder loads first.

What he said.

I fear I have not put my question very well, when doing a powder work up is there any reason tn select an OAL other than the purely arbitrary?

Short answer no, the 2.8" Saami Spec COAL is a good place to start unless you're loading bullets with a profile that dictates otherwise.

For example a round nose 300gr bullet loaded to 2.8" in a .308 would most likely be jammed into the lands preventing bolt closure while a lighter 115gr soupcan bullet would simply fall out of the case neck before getting anywhere near the same COAL.

If its magazine fed then you are restricted to a cartridge over all lentgh that fits your magazine. Once you have developed your load you can start playing with overall lentgh but any thing that will not feed from the magazine will have to be single feed.
You can use a over all lentgh gauge but if they dont fit in the magazine there is no point unless you plan to single feed them.
If you are using 155 grain bullets just use the over all lentgh recomended in the manufactures load data.

If you must use the magazine, as you've found, you need a decent amount of shank in the neck for a firm grip to keep things shipshape under recoil forces and stresses on the bullet as it is pushed up and out of the magazine into the chamber. You actually don't need the full calibre depth as 0.2-0.25" of shank in the neck with a decent amount of tension holds the bullet straight and secure. Actually, with the #2155 SMK being a short bullet, you have under 0.2" of shank in the neck at the SAAMI 2.800" anyway - IIRC the NRA RUAG contract round was loaded to ~2.75" which puts 0.23" of shank in the case-neck.

If you can find your way to single-load rounds, you can manage with still less, around a tenth of an inch in the 308. Over the years, I've had outstanding accuracy in several rifles with several different cartridges - in some cases when using very short / light varmint bullets as with 55s in the 243 Win and 6mm BR Norma seated at a tenth of an inch or less. A tenth of an inch grip, sometimes even less works fine in a carefully made and handled round. You need good ammo boxes like the MTM RM50 and preferably with some expanded foam liner in the lid cavity to protect relatively fragile rounds before they get as far as the rifle.

While the 155gn #2155 SMK is very jump tolerant, it does no harm to seat it out as far as is practicable. If nothing else, it increases the available powder space - no bad thing as you're experimenting with a 23-inch barrel rifle so anything that increases MVs is a good idea, subject to retaining precision. If you get into heavily compressed charges though, short grip and low neck tension isn't a great combination as COALs will vary and in extremis, the bullet may even be pushed back out of the case completely.

As an example of just how little neck you need to get a round to work well, have a look at this, a radical near no-neck .30 BR used in US Hunter Bench Rest competition, the .30 BR Wolf Pup

http://bulletin.accurateshooter.com/?s= ... mit=Search

If you're just touching the lands at a COAL of 2.880", then somewhere around 2.850-2.860" will work fine in single-loading with just over a tenth inch grip. Make up a few rounds at this length and see how they work in practice. If OK, then as you correctly note, it gives you a seating / COAL set-up to leave as a constant while you play with the other variables.

The Lord Flashheart wrote:I have measured to the lands and it is around 2.880" or 2.300"( measured with a Hornady comparator thingy) to the ogive.

The thing is this gives bugger all shank in the case neck and the problem with this was demonstrated when the extracted case rolled off the desk and onto the carpeted floor, visibly knocking the bullet out of true.

As a rule I seat them around 0.015" to 0.020" off the lands subject to a minimum of 0.100" engagement with the neck; this has worked well for the last 25k rounds or so.

..

Laurie wrote:If you must use the magazine, as you've found, you need a decent amount of shank in the neck for a firm grip to keep things shipshape under recoil forces and stresses on the bullet as it is pushed up and out of the magazine into the chamber. You actually don't need the full calibre depth as 0.2-0.25" of shank in the neck with a decent amount of tension holds the bullet straight and secure. Actually, with the #2155 SMK being a short bullet, you have under 0.2" of shank in the neck at the SAAMI 2.800" anyway - IIRC the NRA RUAG contract round was loaded to ~2.75" which puts 0.23" of shank in the case-neck.

If you can find your way to single-load rounds, you can manage with still less, around a tenth of an inch in the 308. Over the years, I've had outstanding accuracy in several rifles with several different cartridges - in some cases when using very short / light varmint bullets as with 55s in the 243 Win and 6mm BR Norma seated at a tenth of an inch or less. A tenth of an inch grip, sometimes even less works fine in a carefully made and handled round. You need good ammo boxes like the MTM RM50 and preferably with some expanded foam liner in the lid cavity to protect relatively fragile rounds before they get as far as the rifle.

While the 155gn #2155 SMK is very jump tolerant, it does no harm to seat it out as far as is practicable. If nothing else, it increases the available powder space - no bad thing as you're experimenting with a 23-inch barrel rifle so anything that increases MVs is a good idea, subject to retaining precision. If you get into heavily compressed charges though, short grip and low neck tension isn't a great combination as COALs will vary and in extremis, the bullet may even be pushed back out of the case completely.

As an example of just how little neck you need to get a round to work well, have a look at this, a radical near no-neck .30 BR used in US Hunter Bench Rest competition, the .30 BR Wolf Pup

http://bulletin.accurateshooter.com/?s= ... mit=Search

If you're just touching the lands at a COAL of 2.880", then somewhere around 2.850-2.860" will work fine in single-loading with just over a tenth inch grip. Make up a few rounds at this length and see how they work in practice. If OK, then as you correctly note, it gives you a seating / COAL set-up to leave as a constant while you play with the other variables.

Finally got to do the shooting.

It would appear that the situation is slightly more complex than "Find the most accurate powder charge, then fiddle with OAL".

This approach is a valid one in most cases and it seems that there is more effect to changing the OAL than the powder charge ( within reason) in terms of grouping on short range targets.

What I found however was that there doesn't appear to be an "optimum" seating depth for a given bullet and chamber that always gives the best results regardless of the powder type used ( note powder type and not most accurate charge of that same powder).

Given that different powders seem to like different OALs at different charge weight it seems that going back to first principles, it's all to do with barrel exit time and that this last is a hybrid of both powder and OAL.

This makes sense, the basic physics would seem to be unassailable but does beg a few questions:

1. In a particular load ( 200 grain accubonds/3006/RL17 powder) I found the most accurate powder charge with the bullet seated out as long as the throat on my Tikka would allow ( 3.440" to the lands,I backed off 2 thou to allow unloading of a unfired cartridge) and couldn't get the groups below and inch and a half with the best ES in velocity being around 60fps and the worst 110fps. I then took that most accurate load ( which also showed the worst ES of 110fps) and reduced the OAL in 20 thou steps until I found the best group ( around 3/4" and 20fps ES) at around 3.320". The strange thing was that the velocity fell as the bullet was seated deeper, equivalent to half a grain of powder less charge at the inital OAL. I am guessing this is due to some sort of freebore effect like an old Weatherby but am wondering what is going on in terms of internal ballistics and how this would relate to better groups and less ES?

2. The 308 load I asked about up here gave best results with an OAL of 2.800 with RL15 ( honest 1/2 MOA for ten shots and ES of 15 fps) whereas the N140 gave best results at 2.850 ( 1 MOA group and ES of 10 ), is this due to barrel time more than anything else? Would it be possible to tune a N140 load to match the RL15 in theory or are they too different? Experimenting with QL indicates that the barrel times can be made very similar, why would this not translate to real world results or is there something else at play here?

Thanks again for all your help and knowledge Laurie, it is very much appreciated.