Velocity question

[velocette]

For my .308 AR rifle. 18" bull barrel. My primary load is 41 gr of IMR 4895 behind a Sierra 168 gr HPBT match bullet in Federal brass. Excellent accuracy, sub moa. (Just below the Sierra manual's max of 41.3) All loads seated to magazine length of 2.8"
Chronographed at 2450 fps by a fellow shooter at my local range.
Now many months later, I do not have a chronograph available & am curious about using brass of larger powder capacity. The Fed case is the heaviest .308 brass (182 gr. average) with the least internal capacity.
If I were to use a case with larger internal capacity such as a Winchester or Hornady (160 grain average) and use the Hornady reloading manual's charge data for a 168 grain bullet which is 2 grains higher for max, is it reasonable to expect a 50 ~ 100 fps increase in velocity? Or would the larger case dimension obviate the difference? Obviously, all loads to be worked up to being observant of pressure signs.

Roger

 

[Steve C]

What will happen with a change in components is unknown. The only way to find out is to get a chrono and try the changes.

Personally if I have a sub MOA load I stick with it. I wouldn't worry about a small gain in velocity. You are just as likely to loose your accuracy as increasing your velocity.

 

[AR15barrels]

Using quickload to model your question, I come up with the following results:

Cartridge          : .308 Win. (52gr water capacity)
Bullet             : .308, 168, Sierra HPBT MatchK 2200
Cartridge O.A.L. L6: 2.800 inch or 71.12 mm
Barrel Length      : 18.0 inch or 457.2 mm
Powder             : IMR 4895

Predicted data by increasing and decreasing the given charge,
incremented in steps of 0.595% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step    Fill. Charge   Vel.  Energy   Pmax   Pmuz  Prop.Burnt B_Time
 %       %    Grains   fps   ft.lbs    psi    psi      %        ms
-02.4  103    41.00   2451    2240   53814   9458     94.2    0.992  ! Near Maximum !

Cartridge          : .308 Win. (55gr water capacity)
Bullet             : .308, 168, Sierra HPBT MatchK 2200
Cartridge O.A.L. L6: 2.800 inch or 71.12 mm
Barrel Length      : 18.0 inch or 457.2 mm
Powder             : IMR 4895

Predicted data by increasing and decreasing the given charge,
incremented in steps of 0.595% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step    Fill. Charge   Vel.  Energy   Pmax   Pmuz  Prop.Burnt B_Time
 %       %    Grains   fps   ft.lbs    psi    psi      %        ms
-02.4   96    41.00   2379    2111   47123   9493     93.1    1.056
-01.8   97    41.25   2393    2137   47967   9558     93.4    1.047
-01.2   97    41.50   2408    2163   48828   9622     93.6    1.038
-00.6   98    41.75   2422    2189   49705   9685     93.9    1.030
+00.0   99    42.00   2437    2215   50599   9748     94.1    1.021
+00.6   99    42.25   2451    2242   51508   9810     94.4    1.013  ! Near Maximum !
+01.2  100    42.50   2466    2268   52432   9871     94.6    1.005  ! Near Maximum !
+01.8  100    42.75   2480    2295   53381   9931     94.9    0.997  ! Near Maximum !
+02.4  101    43.00   2495    2322   54344   9991     95.1    0.989  ! Near Maximum !

Cartridge          : .308 Win. (58gr water capacity)
Bullet             : .308, 168, Sierra HPBT MatchK 2200
Cartridge O.A.L. L6: 2.800 inch or 71.12 mm
Barrel Length      : 18.0 inch or 457.2 mm
Powder             : IMR 4895

Predicted data by increasing and decreasing the given charge,
incremented in steps of 0.61% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step    Fill. Charge   Vel.  Energy   Pmax   Pmuz  Prop.Burnt B_Time
 %       %    Grains   fps   ft.lbs    psi    psi      %        ms
-02.4   91    41.00   2315    1999   42035   9494     92.0    1.116
-01.8   91    41.25   2329    2023   42746   9563     92.3    1.107
-01.2   92    41.50   2343    2047   43470   9631     92.6    1.098
-00.6   92    41.75   2357    2072   44205   9698     92.9    1.090
+00.0   93    42.00   2371    2096   44954   9765     93.1    1.081
+00.6   93    42.25   2384    2121   45715   9831     93.4    1.072
+01.2   94    42.50   2398    2146   46489   9896     93.6    1.064
+01.8   94    42.75   2412    2171   47278   9961     93.9    1.056
+02.4   95    43.00   2426    2196   48079  10025     94.1    1.048
-01.7   96    43.25   2440    2221   48894  10089     94.4    1.039
-01.1   96    43.50   2454    2246   49724  10151     94.6    1.031
-00.6   97    43.75   2468    2272   50569  10213     94.8    1.023
+00.0   97    44.00   2482    2298   51428  10274     95.0    1.015  ! Near Maximum !
+00.6   98    44.25   2496    2323   52301  10335     95.3    1.008  ! Near Maximum !
+01.1   98    44.50   2510    2349   53191  10394     95.5    1.000  ! Near Maximum !
+01.7   99    44.75   2524    2376   54095  10453     95.7    0.992  ! Near Maximum !
+02.3   99    45.00   2537    2402   55018  10511     95.9    0.985  ! Near Maximum !

 

[boommer]

Hey FOR A BUCK @ QUARTER YOU CAN GET a Crony! and learn how to read pressures, in the real world each chamber is different the bore is different every thing has as effect and somethings looks good on paper but in the world of pressures, has to have all volumes and expansions have to be exact, powders have to be exact to what is tested. I'm not saying that load charts and such are not road maps but don't set them in stone.

 

[Grumulkin]

I spent some decades happily reloading without a chronograph. Frequently a different brand of brass will give different results on the target. If using different brass it would be prudent to see if the point of bullet impact and accuracy is similar between them and if not, do some load adjustment.

 

[velocette]

Randall;
Thank you very much for your work. Rather apparently, IMR 4895, no matter what case volume is used results in about the same results.
Do you think if I used Alliant Reloader 15, I could improve the results with larger volume cases?

Thanks
Roger

 

[AR15barrels]

Randall;
Thank you very much for your work. Rather apparently, IMR 4895, no matter what case volume is used results in about the same results.
Do you think if I used Alliant Reloader 15, I could improve the results with larger volume cases?

Thanks
Roger

Actually, there is some gain there, but it is small.

Look at the pressures and velocities in each of the three different case volumes.

At 52gr water capacity and 2450fps, you are at 53,800psi
At 55gr water capacity and 2450fps, you are at 51,500psi
If you bump the charge up to match the pressure, you actually have more velocity by about 35fps.

 

[velocette]

Randall;
Thank you very much. You have answered my questions very well.
Your help is very much appreciated.

Roger