In the last week, I had a shooter have issues with subsonic bullets stabilizing in 5R rifling barrels, along with some previous issues I've seen. The twist rate "on paper" shows the bullet should easily be stable, but bullets are tumbling. Here's why. Traditional rifling is 4 (or 6 ) groove, so you have rifling opposite on each side squeezing the bullet firmly between the rifling and applying force to spin the bullet. This rifling style also has a more sharp angle on the grooves to grip the bullet to spin it. It provides good grip to the bullet for stability.

On 5R barrels (ratchet rifling), there's 3 things happening that are different than traditional rifling. On 5R rifling, their are 5 grooves in the barrel. 1-These grooves do not have a groove on the opposite side which does not "squeeze" or grip the bullet as much. 2- the grooves of the rifling has a more ramped, or sloped angle to the rifling. 3- Many 5R rifling is also angled (not all, but many are).The rifling is taller on one side and shorter on the other. All of these 5R features are great for reducing fouling and allowing the bullet to glide down the barrel with less friction and resistance. But at the same time, it reduces the amount of actual force, or twist, that is applied to the bullet for proper stability.

Keep this in mind if you are calculating twist rates and your bullet stability is in the "marginal" stability factor.

Interesting...I thought that 5R rifling was supposed to have some accuracy advantages, but if what you are saying is correct (I have no reason to doubt you), then it would seem that 5R rifling is not as effective as traditional rifling. So what is the advantage of 5R rifling? and why would one choose a rifle with 5R rifling?

I'm not talking about accuracy, I'm talking purely about bullet stability. 5R rifling has proven itself as a very accurate method of rifling (I even have one myself). I'm only talking about bullet stability, and mainly at the subsonic speeds. Right now, there are many shooters who have suppressors and want to run subsonic rounds. The traditional twist rates work well for 4 and 6 groove barrels. But the 5R rifling is not working as well for proper bullet stability. It simply does not apply as much twist to the bullet for proper stability.

This would also effect your BC to some degree...interesting.

There seems to be a lot at play with stability.

Mildly off topic, but I had looked at buying some 170 grain .277 cal bergers too shoot in my 270 win, but everything I see says they wont stabilize. Reading on the internet... yeah I know the internet.... but have read several people have gotten them to stabilize fine in a 10 twist.

OK, maybe I worded in wrong. When I said not as effective, I was referring to stability. If you lose stability, you lose accuracy too, correct? Would the 5R rifling have the same effect if you are supersonic? As in, "With 5R rifling you may not be able to stabilize the heavy for caliber bullets". Or does it only effect subs?

So it looks like the 5R is more similar to polygonal rifling. Is that correct? Do you know if poly barrels have trouble stabilizing at slower speeds?

My HK has a poly barrel and I like it. Easier to clean for sure.

Unclebubba it’s only 1 example but I had excellent accuracy with an Obermyer 5r and 162 Amax in a 7-08. Best group was 1.5” at 600 yards. That’s also the only time I shot a group at range past 100 yards. I had apparently dialed wrong or the wind shifted on me and I missed 3 shots in a row on a 4” clay. Not my finest moment but I was proud of the group. Take all that for what it’s worth.

I have two, a Remington 700 5R Milspec in .308 and .300 Blackout AR. When I researched these I don't recall seeing the second radius. Also, does not the 5R provide more velocity by keeping more pressure behind the bullet, due greater surface contact between the bullet and bore?

From the FAQ on Boots Obermeyer's web site. http://obermeyerbarrels.com/faq.html



Chad is quite possibly right, I do not disagree at all. At normal operating pressures say 45k and above I would also expect the bullet to Obturate than at subsonic loads pressure. The obituration will cause the bullet to expand to fill those voids and why 5R rifles shooting supersonic Mach 2 or 3 so well.

Just some thoughts

If you lose stability, meaning that the bullet is now tumbling, then yes, you have lost accuracy. If you go with a slower twist rate but the bullet is still stable, the accuracy is what you make of it. By simply going with a slower twist rate and keeping the bullet stable does not decrease or increase your accuracy potential. I can find an accuracy node in a 308 Win with the same bullet in a 1:10" twist and in 1:12" twist. If the bullet is stable, I can get it to shoot (generally speaking).

I just spent the last hour talking with Frank Green at Bartlein barrels about this topic. We discussed a lot of factors. His take on rifling twist rate on factory barrels is 2 part. His is able to measure barrels to very tight tolerances. He has seen factory barrels have slower twist rates (sometimes much slower) than what was listed. Second, the bore and groove dimensions are not as consistent and can be under sized or over sized. He has seen some 308 Win factory barrels have .308" increasing up to .309"+, which causes stability and accuracy issues in itself. The 5R rifling also does have less friction on the bullet going down the barrel.

I have spoken to enough shooters and tested enough rifles in 5R with subsonic ammo, that something is going on with the 5R rifling to cause the bullet to not stabilize a subsonic bullet. I also have a 5R barrel in my 308 Win, and it does fine for regular super sonic ammo. But slow it down to subs, and I don't see the stability of the bullet when the calculated numbers tell you you are good.

This morning, it was 338 Lapua subs with my 300 grain SMK in a Savage 110 BA with a 1:9.3" twist with 5R rifling. This combination shows a 1.65 bullet stability with the Berger calculator. But these bullets are tumbling about half of the time. The calculations show the bullet is plenty stable, but they are not. He's bringing me his rifle tomorrow and I'm going to work up a new load with a 250 grain flat base soft point bullet, to help increase bullet stability. These should work just fine. But we'll see!

I was thinking the same thing. It would be fun to test this idea by using different powders when loading subsonic.

Very interesting Chad. Any correlation to the shape of the groove vs. the number of grooves? I've read somewhere that not all 5R have that same shape groove as the Obermeyer 5-R.

Not to change the topic, but have you seen any difference in accuracy of 5 vs. 4 groove rifling? Not many people have such a variety of rifles as compared to you so I would think you have a better idea than most.

Hey Tim, I think if the rifle is built well with a good barrel and tight tolerances on the ammo, any barrel can shoot very well. I think there are some rifling options that allow for different effects on the bullet. For example, I run a tight bore Obermeyer on my 300 Win Mag. It has a .298" bore and .306" groove, and squeezes the bullet down .002". For the longest time I couldn't figure out why I was shooting much flatter than the listed BC's. The bullet is being squeezed down .002" and is no longer .308" and increases the BC by 4%. There's also some 3 groove barrels that will increase your velocity, but have much reduced barrel life. My match 308 barrel is a 5R, and it's an absolute hammer. I haven't tested any subs in that 5R barrel, due to the threading of that barrel. But I've played with some other 5R's that I've seen issues with stability at the subsonic speeds that should otherwise be 100% stable. But I'm not 100% sure why.

Chad, If you speed those same bullets up to normal supersonic speeds for the caliber do they shoot well and fly pointed end first?

Have you tried different brands or lots of bullets in the suspect 5r barrels? Main reason for that question comes from the short range br world as it seems the light 6mm bullets used in ppc don't fair as well when shot out of a 5r vs conventional rifle barrels.

One other question is are the rifles with issues the same barrels as not all 5r or labeled 5r type are the same?

Hey dee, I'll work up a sub load in a rifle with traditional rifling and test that same load in other rifles to confirm the performance. Those rifles also shoot well. In a given twist rate, I'll calculate the correct length of bullet I need to stay fully stable (or at least on the high side of marginal stability). On this 338 Lapua sub with 300 grain SMK, the customer is local and will be dropping off his rifle to me so I can work up a new load for him with a lighter and shorter 250 grain bullet specific to his gun. He gets a specific load developed for his rifle and I get the data to use and add a new bullet option of 250 grain 338 Lapua subs. It's a win win.

The 5R stability issues in subs I've seen over the few years have all been different makes and brands. It's just happened plenty enough times when a customer calls in about their subsonic bullet not stabilizing. The rifle they have with 5R barrel has plenty fast of twist to stabilize the bullet they are shooting, but the bullet is tumbling or starting to go through paper sideways at 50 or 100 yards. I'll test that specific ammo out, and the velocity be spot on and stable in a different rifle with standard rifling.

Like this Savage 110 BA in 338 Lapua. It has a 1:9.3" twist and 5R rifling. I know from previous rifles that all I need is 1:10" twist to stabilize the 300 grain SMK. I've shot these in several rifles myself, and shipped out many thousands of rounds of 338 subs with 300 grain SMK. The feed back I get on that specific sub load is very good and accurate. Most call me back to tell me the round is so fun to shoot and makes them giggle. (I get it, it does me too!) So, going to a 9.3" twist should spin that bullet even faster for better stability than a 1:10", but it's not there. I'll get his rifle today or tomorrow and go shoot those rounds to verify the speeds and see what's happening.

Hey Kevin, Yes, if I load the same 300 SMK at normal supersonic speeds, they will shoot very well. A buddy of mine has a Savage 110 BA and shoots the 300 SMK in it with good results.

So at standard velocity and pressure for the cartridge they shoot well in 5R or standard rifling but at sub velocity they do not stabilize in the 5-R. And the Sub loads shoot well in standard rifling at even a little slower twist real good.

So you could be right about the pattern of rifling being the culprit, at the reduced velocity and pressure of the subs.

Reminds me of another example above with the polygonal rifling barreled glocks not shooting lead bullets well.

is the velocity lower in the -R barrels that do not shoot it well. Figure you have made sure of that in the 308, but while you have his 338 it might be interesting to find out. Take some jugs of water out and see if you can catch one of those tumbling bullets and see what the rifling engraving looks like if you can, might give some information

^^^ All good questions. I'll know more once I shoot it and see what's going on. He just dropped off the rifle to me today. The velocities are right on par with at about 1020-1050 fps.
The Glock issue with poly rifling and lead or copper coated lead bullets I think is the same issue.