So I have been hearing that the 168 SMK suffers some kind of ballistic problem at longer ranges. So I have been contemplating going to the the 175 SMK but am concerned that my 20 inch barrel will not be able to generate sufficient velocity. How much muzzle velocity is needed at this weight to get out to the 1000 meter mark efficiently?
175 SMK + 20" barrel = low velocity?
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rnbra-shooter
CGN Ultra frequent flyer
- Location
- New Brunswick
Are you sure you need 1000m, and not 1000 yards / 900 metres?
Under what atmospheric conditions will you be shooting (elevation and air temperature)?
Using a 175 SMK in a 20" barrel is a pretty good choice. If it's not quite good enough, there are a couple of other higher performance bullets that could be used.
Under what atmospheric conditions will you be shooting (elevation and air temperature)?
Using a 175 SMK in a 20" barrel is a pretty good choice. If it's not quite good enough, there are a couple of other higher performance bullets that could be used.
1580 ft above sea level. Average temperatures in mid 20's.
Some shooting done at measurements in yards and other "in field" shooting measured typically in meters.
168 SMK are at low 2700's right now but still pushing faster. Found Win 748 works great with the short barrel. Complete burn and very little carbon depositing.
Some shooting done at measurements in yards and other "in field" shooting measured typically in meters.
168 SMK are at low 2700's right now but still pushing faster. Found Win 748 works great with the short barrel. Complete burn and very little carbon depositing.
FYI (and Daniel alludes to this)
Palma, TR and even FTR are sports that shoot 308's to 1000 yards. These guys do it with 154-156 grain bullets. These "palma" style bullets have very good ballistic coefficients, in fact the 155 Sierra Palma Match King has essentually the same BC as a 175 SMK! (difference of .001)
These excellent long bullets will give you a good 150-200 fps more than you are getting, and will yield outstanding long range results.
Palma, TR and even FTR are sports that shoot 308's to 1000 yards. These guys do it with 154-156 grain bullets. These "palma" style bullets have very good ballistic coefficients, in fact the 155 Sierra Palma Match King has essentually the same BC as a 175 SMK! (difference of .001)
These excellent long bullets will give you a good 150-200 fps more than you are getting, and will yield outstanding long range results.
gushulak
CGN frequent flyer
Most factory 175gr. match bullets go 2600-2650 based on my chrony. My handload I use in F-class are doing 2650 out of a 26" barrel and I have no problem getting out past 1000 yards.
168's get iffy once they go subsonic around 900yards. I have seen some guys have great success with them, others frustrated because many keyhole. Depends on the rifle.
With a short barrel of 20" you should probably be pushing the 155gr. match bullets to their limit, this should be enough to get into the game at 1000yards. The extra 6" of barrel really make all the difference at these distances though.
168's get iffy once they go subsonic around 900yards. I have seen some guys have great success with them, others frustrated because many keyhole. Depends on the rifle.
With a short barrel of 20" you should probably be pushing the 155gr. match bullets to their limit, this should be enough to get into the game at 1000yards. The extra 6" of barrel really make all the difference at these distances though.
Absolutely. Run the numbers through any one of a number of on-line ballistics calculators. At a muzzle velocity of 2850 (easily attainable) your 1000 yard velocity will be 1363 fps (still supersonic) and you will have about 31 MOA drop.
If you can get 2900, you will have a 1000 velocity of 1392 fps and have 30
MOA of drop.
This is better than a 6BR using Berger 105 VLD bullets.
FYI, Palma shooters use 30" barrels by convention. (This is more about separating front and rear sights than it is about velocity gains) They typically get MV's near 3000. This ONLY represents a difference of about 60 fps at 1000 yards, so I'm afraid i have to disagree whole-heartedly with Gushulak. I reiterate: the 175 SMK and the 155 Palma MK have the same BC. The same ballistic performance at 150-200 fps faster? Why would you need to push it to the limit??
The numbers I saw on Sierra's sight show the numbers almost equal at 2700+. At less than 2700 the 175 SMK does beat the 155 PMK. How far of distance is covered until the 155 PMK falls below 2700 fps and loses its equal BC?
I am asking this now because I can get a bulk box of 155 PMK's.
How do the 175 and 155 compare at distance?
Obtunded, can you give me a link to a trustworthy ballistic calculator?
I am asking this now because I can get a bulk box of 155 PMK's.
How do the 175 and 155 compare at distance?
Obtunded, can you give me a link to a trustworthy ballistic calculator?
The 20" barrel is the shortest length needed to get full velocity.
I use the JBM site for a calculator: http://www.jbmballistics.com/calculations/calculations.shtml
Rather than use Sierra's semi-questionable G1 BC data, why not use Bryan Litz's test data?
PMK #2155 - G7 BC is 0.214
PMK #2156 - G7 BC is 0.229
175gr. SMK - G7 BC is 0.243
None of these are sensitive to velocity and are more accurate than the fudged G1 based data. Make sure you specify the G7 reference when using whatever ballistic calculator you choose.
Mark
Jamie, have you had problems with the 168's??
I will rebarrel to 26 or better when this one burns out.
rnbra-shooter
CGN Ultra frequent flyer
- Location
- New Brunswick
Ditto what mmatockx said. Use the Litz G7 numbers and be done with it, no mucking about.
Using Jamie's data (2670 fps from a 20" bbl) and your atmospheric data (1580', 25C) and Litz's bullet data, JBM says 1222fps at 1000y and 94.5" of drift in a 10mph crosswind. This wil work fine. If you go back to 1100 yards (about 1000m), you are at 1116fps (just becoming subsonic).
Now if you can get 2670fps with a 175, then with the same pressures you can get 2837fps with a 155.
If you are using the "old 155" (#2155), then your bullet will be arriving at 1000yards at 1176fps and 103" of wind deflection. At 1100 yards the speed is definitely subsonic at 1079fps.
If you are using the "new 155" (#2155), then this bullet arrives at 1000y going 1261fps and 93.0" of wind defection (note that it is equalling/outperforming the 175SMK). It's still going strong at 1100yards at 1141fps.
The bottom line is, a hot-loaded 155 (old or new) or 175 SMK will work fine for you out to 1000 yards. Have at it!
Using Jamie's data (2670 fps from a 20" bbl) and your atmospheric data (1580', 25C) and Litz's bullet data, JBM says 1222fps at 1000y and 94.5" of drift in a 10mph crosswind. This wil work fine. If you go back to 1100 yards (about 1000m), you are at 1116fps (just becoming subsonic).
Now if you can get 2670fps with a 175, then with the same pressures you can get 2837fps with a 155.
If you are using the "old 155" (#2155), then your bullet will be arriving at 1000yards at 1176fps and 103" of wind deflection. At 1100 yards the speed is definitely subsonic at 1079fps.
If you are using the "new 155" (#2155), then this bullet arrives at 1000y going 1261fps and 93.0" of wind defection (note that it is equalling/outperforming the 175SMK). It's still going strong at 1100yards at 1141fps.
The bottom line is, a hot-loaded 155 (old or new) or 175 SMK will work fine for you out to 1000 yards. Have at it!
rnbra-shooter
CGN Ultra frequent flyer
- Location
- New Brunswick
I knew that would happen. 3 bulk boxes all 2155.
That's not necessarily a bad thing at all. They are good bullets, and it is well-known how you get them to shoot really well. They are fully capable of winning a national championship.
If you wanted #2156s, you might be able to work out a 1-for-1 trade for them. Some shooters who have known-good #2155 loads might not care to put in the effort to come up with a similarly good setup for the #2156s (in fact I have a buddy who might fall into exactly this camp).
Maximum velocity... no. But a 20" match barrel is the minimum you need to reliably tune to the even node with the 175 SMK and powders like Varget, RL-15 and 4895. In 20" to 26" barrels, most will tell you the best groups with the 175 are found with velocities in the 2600's. You can hit the odd node and get velocities in the 2700's, but the groups aren't as good. You need at least a 30" barrel to hit the next even node. In a 308, 26" barrel length only gives you added weight and less stiffness with little-to-no velocity advantage over the shorter barrel if you're loading the 175 to the tighter accuracy node.
The few inches of length gives you more velocity argument is meaningless if you aren't shooting factory ammo or adding enough length to be able to hit the next tight accuracy node.
