Best bullet weight for SBR

[johnny mac]

I'm new to the 6.8, and I'm a little confused about what weight works best in shorter (11"-12.5") barrels. My understanding was that the 55gr. 5.56 worked OK out of a 20", but when the 16" and 14.5" got popular heavier bullets were needed to maintain performance at greater range. I am in the process of registering a couple of SBR's, and I'm trying to figure out the optimal load/weight. I understand the basics of ballistics and ballisitc coefficient, but just barely. From reading posts here it seems that the lighter bullets (85-90gr.) are popular for hogs, which have a reputation for being tough, and often at least man-sized if not larger, so it would seem that a hog load would be plenty good for deer and even black bear. However, it also seems like a lot of hogs are shot close-do things change at greater distance? What would you guys consider max effective range for an 11" gun, and would trajectory/retained energy be significantly different from 85gr. to a 110/115, or even 130? Most ballistic info seems to be for 16" bbls. Thanks.

[HTR]

Welcome to the board. You have some good questions. I believe that one might have to think a little differently with regard to 6.8 SBR's versus other calibers. This is because they achieve such high velocity in short barrels.

As for the BC's they are all so similar across styles and weights of bullet, that it has become standard practice to go with lighter bullets. This is where the "new" thinking comes in. If 77 grains was "what was needed" to finally make a .22 caliber rifle "effective," then anything heavier would be better, especially if it is going faster. Not only is an 85 grain bullet heavier than one in 77 grains, but the huge difference comes in when you factor in the velocity achieved with the 6.8's parent case. If you shoot the 85 grain Barnes TSX loaded over 31 grains of RE7, or perhaps you use the SSA tactical load, you are likely to see around 2900 FPS in the 11 or 12" barrel. That beats the 5.56 / 77 grain SMK by almost 400 FPS in the same barrel length.

So, if you can get that bullet to 2900 FPS, you will still have 1000 Foot-pounds of energy at around 200 yards. I can tell you that I have dropped two hogs in the 250- 350 pound range in the last week, using this load, at 207, and then 167 yards....and, I mean D. R. T.

If you used the 130 grain bullet, you will retain more energy but the velocity will likely only be in the 2350 range at the muzzle. That just doesn't give me the trajectory I like to have. If you can't hit the target in the vital zone, I could care less how much energy the bullet is carrying at 200 yards. I wlould choose the flatter shooting bullet. Faster from the muzzle means flatter at the ranges that are realistic for 6.8 SBR's, i.e, around 250 yards and under.

[johnny mac]

Wow, I got lucky fast on this post! HTR, I was hoping you would be one of the ones to answer. I"ve really enjoyed your posts, and they've been a big factor in warming me up to the 6.8. I have a Robinson XCR, and I kind of fell for the 6.5 Grendel. The XCR was supposed to be available in 6.5 some time ago, but it still is not. I got 1,000 rounds of Wolg 6.5 for a little over $10/box-also part of why it was more appealing than 6.8 initially.
I've given up on waiting for the 6.5, since the 6.8 is available now, and I've gotten more realistic about my own needs and abilities. The 6.5 seems to be equal to the 6.8 in close, and has a real advantage at longer range, but I probably shouldn't be shooting at anything farther than 250 anyway. I've been looking for an all-around gun, personal defense, varmint, deer, etc. and I think 6.8 will do the trick. I aslo read your thread on subsonic loads-I plan on running a suppressor as much as possible. The XCR is piston-driven, and I have an AAC 5.56 can in process that I am going to see if AAC can modify to 6.8, and I have one of ther 7.62 cans on backorder. Do you think the piston and 7.62 can will minimize the pressure issues you had suppressed?

[Clint]

I'm new to the 6.8, and I'm a little confused about what weight works best in shorter (11"-12.5") barrels. My understanding was that the 55gr. 5.56 worked OK out of a 20", but when the 16" and 14.5" got popular heavier bullets were needed to maintain performance at greater range. I am in the process of registering a couple of SBR's, and I'm trying to figure out the optimal load/weight. I understand the basics of ballistics and ballisitc coefficient, but just barely. From reading posts here it seems that the lighter bullets (85-90gr.) are popular for hogs, which have a reputation for being tough, and often at least man-sized if not larger, so it would seem that a hog load would be plenty good for deer and even black bear. However, it also seems like a lot of hogs are shot close-do things change at greater distance? What would you guys consider max effective range for an 11" gun, and would trajectory/retained energy be significantly different from 85gr. to a 110/115, or even 130? Most ballistic info seems to be for 16" bbls. Thanks.

Welcome Johnny,

I'll try to shed some more light on this.
The reason that the 556 worked so well initially was it caused extra damage due to fragmentation. You could almost stay it relied on it.

Found a quote from RCRANDALL that explains things nicely.

rcrandall
68Forums.com Team Member

The 5.56 M855 62 grain FMJBT is the current issue round for the U.S. Military. The fragmentation threshold for that bullet is under 2700 fps. The muzzle velocity of the 14.5" M4 with this ammunition is 2900 fps. Running the numbers on JBM trajectories puts the velocity under 2700 fps somewhere around 80 to 85 yards (using 4000 ft elevation).

The SSA 115 grain OTM has been deemed legal to use by the JAG because it performs like a FMJBT when it hits ballistics gel, it yaws between one and two inches after penetration and breaks (fragements). This round chronographs at 2500 fps muzzle velocity from my 14.7" barrel and has a fragementation threshold of 1900 fps. Running the numbers on JBM the velocity falls below 1900 fps around 270 - 280 yards.
__________________
6.8 Convert

The M855 is spec'ed at 3050 fps out of a 20" barrel.
I don't have all the numbers available, but that's 150 fps faster than the 14.5", making the fragmentation range roughly 150 yards ( I'm guessing ). 150 yards is marginally workable, 85 yards is not.

Without fragmentation, the tiny FMJ bullets, which are not designed to expand, would often just pass through leaving a nice little .22 caliber hole. Hardly DRT.

The frag range of the 77 grain is only 75 yards or so IIRC, but that bullet is long enough and heavy enough to have good effect with its expanding OPEN TIP. This OTM construction puts it on par with other top shelf expanding 556 ammo such as federal tactical bonded and the like. To answer my own question, that is also why Adam likes them.



So summarize some terminal performance on light targets, a non expanding bullet does minor damage, an expanding bullet does probably twice as much damage, and a bullet with enough velocity to fragment might do twice again as much damage.

Also, for best all around performance, you shouldn't rely on fragmentation, but treat it only as a bonus when you get it.

Now the physics of short barrels say that given the same pressure, a lighter bullet accelerates faster.
Also, given the same pressure, a larger bore diameter accelerates a bullet faster.

This is why most pistols are 9mm,10mm(.40) and 11.5mm(.45) in diameter. To get the job done in 4" of length.

As Chris mentioned, In order for an SBR to maintain a rifle like trajectory it needs a fast bullet, helped by both light weight and large diameter. This is where the 6.8 shines vs 556, much higher initial velocity for any given weight.

For the lighter weight bullets being favored on hogs, I'll say that one is tricky. If the TSX bullets never existed, I"ll bet everyone would be using 110gr accubonds, or something similarly heavy that works conventionally. The heavier weight and stouter bullet construction is conventionally required to get adequate penetration and retained weight on heavy game. Here a fragmenting bullet is a looser, because it lacks penetration.

The TSX is really something unique, offering all three things, deep penetration, full expansion and 100% weight retention. It does not fragment unless propelled at Magnum velocities 3400 fps+, I think, and end even then it just looses the front petals and retains the 60% weight of the base. Because of this, the ultra fast 85 TSX just smashes hogs. In the case of the 85TSX vs the 110TSX, the extra velocity seems to be more effective than the extra weight.

[ccoker]

HTR
have you found the 85g Barnes to create bigger wound channels than the 110g TTSX?

I have only shot one hog with the 85 and it punched clean through it

my son shot a pig at 50 yards through the shoulder with an 85g barnes (non tipped) at 3280FPS out of a 243
about a nickel sized exit wound but the heart was completely pulverized

[HTR]

Welcome Johnny,

I'll try to shed some more light on this.
The reason that the 556 worked so well initially was it caused extra damage due to fragmentation. You could almost stay it relied on it.

Found a quote from RCRANDALL that explains things nicely.

rcrandall
68Forums.com Team Member

The 5.56 M855 62 grain FMJBT is the current issue round for the U.S. Military. The fragmentation threshold for that bullet is under 2700 fps. The muzzle velocity of the 14.5" M4 with this ammunition is 2900 fps. Running the numbers on JBM trajectories puts the velocity under 2700 fps somewhere around 80 to 85 yards (using 4000 ft elevation).

The SSA 115 grain OTM has been deemed legal to use by the JAG because it performs like a FMJBT when it hits ballistics gel, it yaws between one and two inches after penetration and breaks (fragements). This round chronographs at 2500 fps muzzle velocity from my 14.7" barrel and has a fragementation threshold of 1900 fps. Running the numbers on JBM the velocity falls below 1900 fps around 270 - 280 yards.
__________________
6.8 Convert

The M855 is spec'ed at 3050 fps out of a 20" barrel.
I don't have all the numbers available, but that's 150 fps faster than the 14.5", making the fragmentation range roughly 150 yards ( I'm guessing ). 150 yards is marginally workable, 85 yards is not.

Without fragmentation, the tiny FMJ bullets, which are not designed to expand, would often just pass through leaving a nice little .22 caliber hole. Hardly DRT.

The frag range of the 77 grain is only 75 yards or so IIRC, but that bullet is long enough and heavy enough to have good effect with its expanding OPEN TIP. This OTM construction puts it on par with other top shelf expanding 556 ammo such as federal tactical bonded and the like. To answer my own question, that is also why Adam likes them.



So summarize some terminal performance on light targets, a non expanding bullet does minor damage, an expanding bullet does probably twice as much damage, and a bullet with enough velocity to fragment might do twice again as much damage.

Also, for best all around performance, you shouldn't rely on fragmentation, but treat it only as a bonus when you get it.

Now the physics of short barrels say that given the same pressure, a lighter bullet accelerates faster.
Also, given the same pressure, a larger bore diameter accelerates a bullet faster.

This is why most pistols are 9mm,10mm(.40) and 11.5mm(.45) in diameter. To get the job done in 4" of length.

As Chris mentioned, In order for an SBR to maintain a rifle like trajectory it needs a fast bullet, helped by both light weight and large diameter. This is where the 6.8 shines vs 556, much higher initial velocity for any given weight.

For the lighter weight bullets being favored on hogs, I'll say that one is tricky. If the TSX bullets never existed, I"ll bet everyone would be using 110gr accubonds, or something similarly heavy that works conventionally. The heavier weight and stouter bullet construction is conventionally required to get adequate penetration and retained weight on heavy game. Here a fragmenting bullet is a looser, because it lacks penetration.

The TSX is really something unique, offering all three things, deep penetration, full expansion and 100% weight retention. It does not fragment unless propelled at Magnum velocities 3400 fps+, I think, and end even then it just looses the front petals and retains the 60% weight of the base. Because of this, the ultra fast 85 TSX just smashes hogs. In the case of the 85TSX vs the 110TSX, the extra velocity seems to be more effective than the extra weight.

This is an excellent post, Clint. You've summed up the TSX as well as I ever could. That is why I love this bullet for hogs. In the 6.8, the velocity seems to be just about perfect for the best bullet performance. I.e, 2900- 3100 FPS.

I get compelte pass throughs, but the energy transfer is enough to literally knock 200-300 pound pigs off their feet. I have never seen anything else quite like it in an AR-15 based cartridge.

Coker....I get the same exit wounds in general. It is usually around the size of a nickel or quarter depending upon angle, hitting of bone, animal, range, etc.

[HTR]

Wow, I got lucky fast on this post! HTR, I was hoping you would be one of the ones to answer. I"ve really enjoyed your posts, and they've been a big factor in warming me up to the 6.8. I have a Robinson XCR, and I kind of fell for the 6.5 Grendel. The XCR was supposed to be available in 6.5 some time ago, but it still is not. I got 1,000 rounds of Wolg 6.5 for a little over $10/box-also part of why it was more appealing than 6.8 initially.
I've given up on waiting for the 6.5, since the 6.8 is available now, and I've gotten more realistic about my own needs and abilities. The 6.5 seems to be equal to the 6.8 in close, and has a real advantage at longer range, but I probably shouldn't be shooting at anything farther than 250 anyway. I've been looking for an all-around gun, personal defense, varmint, deer, etc. and I think 6.8 will do the trick. I aslo read your thread on subsonic loads-I plan on running a suppressor as much as possible. The XCR is piston-driven, and I have an AAC 5.56 can in process that I am going to see if AAC can modify to 6.8, and I have one of ther 7.62 cans on backorder. Do you think the piston and 7.62 can will minimize the pressure issues you had suppressed?

That is going to be an awesome rifle. The can will increase back pressure slightly. To which pressure issues are you referring?

[ccoker]

cool
growing up hunting in South Texas I was brought up on the "light and fast" mindset
the one hog I hit with the 85 was at night and even though the shot was too far back, it only made it about 60 yards
and was a complete pass through.. it wasn't a good clean hit and so we didn't persue it for fear of it being pushed and running far.. we went back the next morning and found it, so, I didn't open it up to see the damage

I think I am going to focus on the 85s for a bit

[BWilson]

I'm having trouble coming to grips with this little bullet thing..................I'm used to 225gr bullets in my .358 Win and 286gr bullets in my 9.3x62

Even 130gr seem tiny to me (-:

But I have to say so far we have killed hogs cleanly on the ranch with 85-130gr 6.8s without problems, so there may be something to this little fast bullet thing????

It does seem that anyting I shoot a Barnes TSX or TTSX (no matter the caliber) into just dies quicky.................

[marinesg1012]

These little bullets work so well becuase they retain all of thier weight you figure with a 130 grain bullet after it hits it may only be a 100 grain or less projectile, with those 85's you are getting the full 85 every time, and I also think them being longer helps them act like a larger bullet..... all I know is they hit stuff and keep going and kill whatever they hit.