OAL and Velocity...

[swerv512]

Just curious how much velocity change you've seen from changing bullet seating depth. I know that a bullet seated closer to the lands can improve accuracy, but i'm more curious about the velocity change. I'm going to attempt to seat my bullets further out (limited by magazine use) to see if i can squeeze out a lil more accuracy. I'm at 2.81 now and plan to head up to 2.87 I assume that seating a bullet further out in the case would increase the internal volume of the case (consequently decreasing pressure- boyle's law)- so tell me some real world examples...

[ar15barrels]

I can run. Quicklosr predictions, but what happens if you run into the throat is that pressure jumps way up real quick.
Quickload can predict for that as well as long as you specify that you are breech seating.
Give m all your details like bullet, barrel length and powder charge...

[Jonathan Doe]

I load at less than maximum powder charge. And, seat the bullets to .005"-.010" from the rifling. Never had any problem and got good accuracy.

[diginit]

If you are crimping into the cannelure, You're kinda limited. You may want to try using a longer bullet. Like a V-Max. Keeping the seating depth and getting closer to the lands. I've had good results with the 55gr. Most rifles seem to like alittle bit of a jump to the lands. I tend to cheat and use military loads to make my comparisons, since they have already done the research.

[swerv512]

Thanks Randall!
.308win
24" barrel
45.0gr IMR 4064
175SMK
2.005" trim
currently seating at 2.81 base to tip (2.230 to ogive)
wanting to go up to 2.87 base to tip (magazine liimited)...
i almost always shoot at DA less than 500- closer to 0 most days

[ar15barrels]

Cartridge          : .308 Win.
Bullet             : .308, 175, Sierra HPBT MatchK 2275
Cartridge O.A.L. L6: 2.810 inch or 71.37 mm
Barrel Length      : 24.0 inch or 609.6 mm
Powder             : ? IMR 4064 ?, Temperature: 50 °F

Predicted data by increasing and decreasing the given charge,
incremented in steps of 0.222% 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

-02.2  105    44.00   2665    2760   58629   7259     98.9    1.177  ! Near Maximum !
-02.0  106    44.10   2670    2771   59049   7270     98.9    1.173  ! Near Maximum !
-01.8  106    44.20   2676    2783   59473   7281     99.0    1.170  !DANGEROUS LOAD-DO NOT USE!
-01.6  106    44.30   2682    2794   59900   7292     99.0    1.166  !DANGEROUS LOAD-DO NOT USE!
-01.3  106    44.40   2687    2806   60330   7303     99.0    1.163  !DANGEROUS LOAD-DO NOT USE!
-01.1  106    44.50   2693    2818   60764   7314     99.1    1.159  !DANGEROUS LOAD-DO NOT USE!
-00.9  107    44.60   2698    2829   61201   7324     99.1    1.155  !DANGEROUS LOAD-DO NOT USE!
-00.7  107    44.70   2704    2841   61641   7335     99.2    1.152  !DANGEROUS LOAD-DO NOT USE!
-00.4  107    44.80   2710    2853   62084   7345     99.2    1.148  !DANGEROUS LOAD-DO NOT USE!
-00.2  107    44.90   2715    2865   62531   7355     99.2    1.145  !DANGEROUS LOAD-DO NOT USE!
+00.0  108    45.00   2721    2876   62982   7365     99.3    1.141  !DANGEROUS LOAD-DO NOT USE!
+00.2  108    45.10   2726    2888   63436   7375     99.3    1.138  !DANGEROUS LOAD-DO NOT USE!
+00.4  108    45.20   2732    2900   63893   7385     99.3    1.135  !DANGEROUS LOAD-DO NOT USE!
+00.7  108    45.30   2737    2912   64354   7395     99.4    1.131  !DANGEROUS LOAD-DO NOT USE!
+00.9  109    45.40   2743    2923   64818   7404     99.4    1.128  !DANGEROUS LOAD-DO NOT USE!
+01.1  109    45.50   2748    2935   65286   7413     99.4    1.124  !DANGEROUS LOAD-DO NOT USE!

Results caused by ± 3% powder lot-to-lot burning rate variation using nominal charge
Data for burning rate increased by 3% relative to nominal value:
+Ba    108    45.00   2755    2949   66355   7309     99.8    1.118  !DANGEROUS LOAD-DO NOT USE!
Data for burning rate decreased by 3% relative to nominal value:
-Ba    108    45.00   2682    2796   59636   7390     98.4    1.167  !DANGEROUS LOAD-DO NOT USE!

Cartridge          : .308 Win.
Bullet             : .308, 175, Sierra HPBT MatchK 2275
Cartridge O.A.L. L6: 2.870 inch or 72.90 mm
Barrel Length      : 24.0 inch or 609.6 mm
Powder             : ? IMR 4064 ?, Temperature: 50 °F

Predicted data by increasing and decreasing the given charge,
incremented in steps of 0.222% 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

-02.2  103    44.00   2639    2706   55800   7324     98.7    1.200  ! Near Maximum !
-02.0  103    44.10   2644    2717   56190   7336     98.7    1.196  ! Near Maximum !
-01.8  103    44.20   2650    2729   56582   7348     98.8    1.193  ! Near Maximum !
-01.6  104    44.30   2655    2740   56978   7360     98.8    1.189  ! Near Maximum !
-01.3  104    44.40   2661    2751   57377   7371     98.9    1.185  ! Near Maximum !
-01.1  104    44.50   2666    2763   57778   7383     98.9    1.182  ! Near Maximum !
-00.9  104    44.60   2672    2774   58182   7394     98.9    1.178  ! Near Maximum !
-00.7  104    44.70   2677    2785   58590   7406     99.0    1.175  ! Near Maximum !
-00.4  105    44.80   2683    2797   59000   7417     99.0    1.171  ! Near Maximum !
-00.2  105    44.90   2688    2808   59413   7428     99.1    1.168  ! Near Maximum !
+00.0  105    45.00   2694    2820   59830   7438     99.1    1.164  !DANGEROUS LOAD-DO NOT USE!
+00.2  105    45.10   2699    2831   60249   7449     99.2    1.161  !DANGEROUS LOAD-DO NOT USE!
+00.4  106    45.20   2705    2843   60672   7460     99.2    1.157  !DANGEROUS LOAD-DO NOT USE!
+00.7  106    45.30   2710    2854   61097   7470     99.2    1.154  !DANGEROUS LOAD-DO NOT USE!
+00.9  106    45.40   2716    2866   61526   7481     99.3    1.150  !DANGEROUS LOAD-DO NOT USE!
+01.1  106    45.50   2721    2878   61958   7491     99.3    1.147  !DANGEROUS LOAD-DO NOT USE!

Results caused by ± 3% powder lot-to-lot burning rate variation using nominal charge
Data for burning rate increased by 3% relative to nominal value:
+Ba    105    45.00   2729    2893   63015   7391     99.7    1.140  !DANGEROUS LOAD-DO NOT USE!
Data for burning rate decreased by 3% relative to nominal value:
-Ba    105    45.00   2655    2740   56670   7454     98.2    1.191  ! Near Maximum !

[swerv512]

Thanks Randall. Gives me some good insight. i guess it's time to load some of those 2.870s up and put them through the chrono and see if the potential accuracy gain is worth the fps loss. i guess i could always load them longer and pump up the powder, but probably not since pressure's already up there...