220 gr SMK 308 Load

T

The Old Salt

New member
Picked them at the Sierra factory store. Wasn't aware that there weren't any loads for them in my manuals. So if any of you have ever worked up a load for 220gr SMKs in a 308 that you would share, I'de appreciate it. Wonder if the Sierra ballistic software has a ballistic curve for it?
 
Usually the .308, being shorter than .30-06, is not loaded with bullets heavier than 200 grains. Part of the reason is that longer bullets just use up too much of the powder space. Another is the SAAMI standard barrel twist for .308 is 12", where for 30-06 it is 10", which makes it a lot easier to stabilize long bullets well. The 10" twist in the '06 was taken from the 30-40 Krag barrel twist which was meant for 220 grain RN bullets originally. In Don Miller's stability estimator, at 2300 fps, which is about as much as I think you could get from a .308 with this big bullet, a 10" twist provides a nice gyroscopic stability factor of about 1.67 in ICAO standard conditions, while a 12" twist only provides about 1.16. It is generally recommended by Sierra not to go below 1.3. The bullets may not be outright unstable, but they tend not to group well in that situation.

So, this is clearly a good excuse to get the 30-06 you always wanted anyway.
 
In 2003 i got blemished Hornady 220 gr round nosed bullets from Lock Stock and barrel. I worked up 30-30 until the brass flowed. I was shooting into the dirt, and never thought about twist rate.

Then I built a rifle, and looking for ways to use up those bullets, I made dummies and checked feeding. Then I want hunting with Barnes pointed bullets. I wound up hunting single shot, as the pointed tips could get hung up where the round nose would not.
 
I would start with Varget. I don't know any one using that heavy of a bullet in the 308 Win, but a lot of shooters use the 185 Berger with Varget. If you have IMR 4064 might try that, but you won't get the velocity that you will with Varget. I don't know if IMR 4350 would give good results. About all the IMR 4350 I can get in a 308 case is 46.5 to 47 grs. Try shoving a 220 grain bullet in the case and you might have to cut back the charge to 44 to 45 grains and that won't push that bullet very fast.
 
Frankly I don't think using a 220gr bullet in a .308 will give either accuracy or the velocity needed for anything other than punching holes in paper. My .02!! William
 
Why are so many 308s in 1-10?
Click to expand...

1, Because bullet length determines how tight or loose a barrel twist rate needs to be. A 1:10 will handle just about anything with the exception of some very heavy subsonic loads (such as the 240 SMK, not going to be good in the accuracy department).

2, the barrel makers were already making 1:10 twist barrels in 30 cal. Same reason why so many non American twists are 1:9.45" even through WWII.

But, a 1:12 twist was the standard military twist, with the M24 sniper rifle using a 1:11.25 5R twist.

And I know I'm not Unclenick, but I hope this is helpful.

Jimro
 
Thanks, it does seem odd that if the 1-12 is intended for 308 that it not be used.

Not that I mind, just seems strange not to follow specifications, millions of ARs made with varying twists and by small mfgs that would seem to be more of a burden on.
 
Twist rate is always a fun topic.

Faster twist can stabilize longer bullets at slower velocities (why some 300 Blackouts have a really tight twist like 1:7). Slower twist reduces the amount of accuracy loss due to bullet imbalance (why 30 BR barrels can have a 1:16 to 1:18). Both the Blackout and the 30BR are the same caliber.

In the middle, there isn't much difference between a 10 and 12 twist for the 308 until you try to do something out of the ordinary (long range high velocity cast bullets for example), then twist rate becomes more of an issue. Or as one forum member was attempting, to launch a 220gr bullet to get the same terminal ballistic effect on beached whales with a 308 as an old 303 British round that was discontinued decades ago.

But as to the reason why the twist rate of 1:10 is popular in 30 cal, it's because of the original round nosed 30-40 Krag load. The twist was that tight to stabilize that long bullet. When the 30-03 was adopted it fired essentially the same bullet, but much faster so the 1:10 twist was retained for the 30-06.

So despite the "new" 308 Win being spec'd for a 1:12 twist, there was an entire industry built around 1:10 twist barrels. Cheaper just to chamber the blanks that you are already making than to retool the rifling.

And if you think that's a fun story, look up how a horses rear end impacted the US space program....

Jimro
 
Jimro,

I think you got it right. Lots of 10" blanks out there. 12" should be just fine for the 168 grain SMK in normal match conditions. I don't actually know how the designers came up with 11.25" for the M24, but expect considerations for extreme cold temperature conditions and measured cold temperature velocities achieved with the 175 grain SMK may have resulted in that number.

It seems to me I read that one of the military marksmanship units had decided an 11" twist was better than 12", but I don't know under what conditions. It would probably have either been for the M1 Type bullet in M118, or they were still experimenting with 180 grain bullets for slow fire at that point.
 
Unclenick,

As far as I can determine, the 1:11.25 twist on the M24 was because that's what Boots Obermeyer provided Remington for the M24 trials. But there is more to it than that. At the time the M118SB load was the standard sniper load for the M21 (M14 based sniper rifle), but Remington originally thought the M24 should be in 30-06. If a "new" 30-06 load was going to be developed with the 180gr or 190gr Matchking, the slightly tighter twist makes a lot of sense. But, at the time there wasn't enough of any single lot of 30-06 match loads in the inventory to complete the sniper rifle trials in 30-06, so the rifles with their long action were chambered in 308 and the trials proceeded with M118SB ammunition.

The M118SB load shot fine through the M21 (and M25) sniper rifles with 1:12 twist, so the only logical reason is that either Boots Obermeyer convinced Remington a slightly tighter twist was better, or that Remington was anticipating a longer bullet for the next generation of sniper rifle.

Incidentally the commercial Browning BAR hunting rifle in 300 Win Mag was also considered during the sniper rifle trials, and if you ever get to the museum at Fort Benning it is sometimes on display. As far as I know, that rifle used a 1:10 twist.

Anyways, fast forward to 2009, and people insist that Remington planned the M24 platform to grow because it used the long action and the Army was looking to transition to the M2010 platform with 300 Win Mag as the round. The 1:11.25 twist proved adequate for stabilizing the Mk248 Mod0 load (190gr SMK) and later the Mk248 Mod1 (220gr SMK) so the transition happened without a hitch, and the Army got to purchase Remingtons "Modular Sniper Rifle" (MSR) which gives swap barrel ability so snipers can train on M118LR or Mk316 Mod0 for cheap, then switch to 300 Win Mag or 338 Lapua (or Norma) Magnum if needed.

We just might be in the last two or three decades of serious sniper training. Lots of folks are looking at the DARPA guided bullet tech and "TrackingPoint" scopes and crunching the numbers about whether it is worthwhile to buy gear that anyone can use, or train snipers to shoot long range with gear that not just anyone can use. When the math is firmly on the side of technology, that's when I expect widespread adoption. It's just too hard to justify an 8,000 dollar rifle and a few hundred thousand dollars in training costs when you can buy a 2,000 dollar rifle and 10,000 dollar scope but avoid the additional training costs per shooter.

Jimro
 
Wow! What a discussion. Been to busy to reply, but thx for all the info. Will try to absorb and may load a few just to see what happens. My barrel will push a 175 SMK at IVs above 2900 fps with a std max load of 45.6g IMR 4064. That said I might do better than the 2300 fps suggested here with the 220 SMKs. Will post up results when I get around to this one. Thx again to all.
 
Unclinick you were right. Yesterday's IV Max was 2326 fps delivered from 38.5g IMF 4064. Might get more powder stuffed in. Varget or CFE223 may produce a higher IV. Group size was in the 0.6" to 0.8"" groups @ 100 yards. That large of groups from my barrel may be more a result of a very dirty barrel and my poor shooting skills.
 
My .02 is you need a much slower powder for that heavy a bullet . IMR 4350 , RL-17 , N-550 and the like would/should do much better .
 
Last edited:
I'm running 208 grain AMAXs in mine, however it's a custom built setup in a long action. So case capacity and seating length aren't an issue. In fact I had it throated out specifically for the 208's.

I can't remember the exact charge but I think it's around mid 40 grain and it is RL17 powder. I started w RL15, but couldn't push as much velocity with it and knew that RL17 would get me there. With a 26" barrel and this load I'm running right around 2530 fps. I have pushed it to 47 grains and a hair over 2600 fps, but began to see pressure signs and backed off.

I can dig the specifics up for you if you're interested. It may be helpful in getting you a starting point for your 220's.


Sent from my iPhone using Tapatalk
 
A quick question and sorry if this seems a hijack, but with respect to 200ish grain bullets in .308 do you encompass a "hotter" overall burn with the heavy bullets vs say 168's. I ask this because I have a R700 heavy 26" 1-12" twist and It loves 155's and 168's. My LGS reloading salesman has the same exact rifle and he swears by the 190 smk ? So is it fair to assume the heavy 200 gr bullets will shorten your barrel life respectively to 168's ?
 
Others can correct me if I'm wrong but I'd say If there were to be a difference the load that holds the larger charge is the one that reduces barrel life . There is a mathematical equation that determines barrel life . It has to do with bore diameter and powder charge . Sorry I don't know what that is . I don't believe it has anything specific to do with bullet weight .

There are other factors like how fast you shoot the next shot resulting in heating the barrel . The hotter the barrel gets and stays as you're shooting . The quicker the throat erodes .

In general the 308 is good for many thousands of rounds . I hear 5k+ before any real noticeable drop off unless you and your rifle are world class shooters . This number concerns me because I just passed the 3k mark on one of mine and it only took me 2-1/2 years to get there . The last time I checked my throat had eroded .060 . I believe that was around the 2500 round count . FWIW 95% of bullet weights have been 175 & 178
 
Last edited:
Road_Clam said:
A quick question and sorry if this seems a hijack, but with respect to 200ish grain bullets in .308 do you encompass a "hotter" overall burn with the heavy bullets vs say 168's. I ask this because I have a R700 heavy 26" 1-12" twist and It loves 155's and 168's. My LGS reloading salesman has the same exact rifle and he swears by the 190 smk ? So is it fair to assume the heavy 200 gr bullets will shorten your barrel life respectively to 168's ?
Click to expand...



Good question. I have no idea, but I'd tend to think a lighter faster bullet would cause more issues with erosion (I'm thinking of my .204 and 22-250). Maybe I'm wrong.

At the end of the day I built mine to eat 208's and if it lasts 5k or 10k rounds really makes no difference, I'll be happy either way :).

I think a lot of people would be very surprised with the ballistic numbers of a .308 with 208's, the numbers are rather surprising. Sure it's no 300 Win Mag, but it's just a nice enjoyable round to shoot.
 
Metal God: Good reason to get a Savage and a wrench. Quick off and on.

I suspect I will just get mine reamed out to 30-06 and shoot it single (which I do anyway, the throw is long enough to extract.

At some point an all new barrel.

Same issue, I am down to shooting one bun, 100 rounds a week at 52 weeks a year........

Well, range is closed for a month, I don't make all the weeks either but as you can see it adds up fast.
 
You must log in or register to reply here.
Back
Top