Lapua Flash Hole Test

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  • Boostmeister

    Lapua Flash Hole Test

    I managed to break several decapping pins on some Alexander Arms and Lapua brass before I found that they are using a smaller than normal flash hole. Roughly speaking, large flash holes are about .080, and small flash holes are around .060. Using a small decapping pin, I found that I was breaking them with a high degree of frequency, and even though I was able to push the spent primer out, there was a noticeable interference fit. Looking around the internet (always risky), I found that the Lapua flash hole is around .057 or .058. I called RCBS and spoke with a Mr. Meyers. What a guy! He sent me replacement pins for the ones I had broken, and explained that while they don't make a specific decap die for the smaller than normal Lapua flash hole, he would make up something using a warranty returned 7mm seating die. All this for no charge. That's Customer Service!!! I continued to look into the reasoning for the Lapua flash hole size and found several posts from a gentleman that worked for Lapua. This individual indicated that the smaller flash hole contributes to a better flame front, lower standard deviation and extreme spreads. At this point I began to wonder how much difference there would be, and why everyone hasn't gone to .058 flash holes. I took 20 pieces of Lapua brass and used a Sinclair flash hole deburring tool to open up the flash holes to roughly .080 (measured at .078). I decapped, resized (setting the shoulder back .002), and cleaned the standard and modified brass. Both samples were loaded with 28.3 gr of Varget (soft load), and then seated a 123 gr Lapua Scenar bullet to an OAL of 2.25 inches. At this point I headed to the range with the reloaded small (stock) and modified flash hole brass, plus a handful of Alexander Arms 123 gr Lapua Scenar ammo (control group). I set up the chronograph 10 feet from the bench, and set up 5 targets at 100 yards. The first target was for the factory ammo. The other four were labeled Small Upper Left, Large Upper Right, Small Lower Left, and Large Lower Right. I then fired eight factory rounds and recorded the velocity. To minimize any barrel heating impact, I would fire a small (factory Lapua flash hole) round at the upper left target, followed with a modified flash hole at the upper right target, then small at the lower left target, and then a modified round at the lower right target. Then repeated that sequence for all remaining handloads. The following were my results:

    Alexander Arms
    Std Dev 29.3
    ES 72.5
    Avg 2567.5

    Standard Flash Hole Lapua Brass
    Std Dev 25.6
    ES 77.5
    Avg 2364.7

    Modified Flash Hole Lapua Brass
    Std Dev 25.1
    ES 82.8
    Avg 2368.9

    In my opinion, the size of the flash hole doesn't seem to make any real impact. While I didn't take any photos of the groups, or measure them for size (not the intent of the test), the target was an orange target spot 1" in diameter. All groups were well under an inch in size. There was some obvious difference in point of impact which I attribute to the 200 FPS difference in the factory ammo over the handloads. I may rerun the test one more time as a check and balance, but pending some widely differing results, I plan to open up the flash holes on my remaining Lapua brass to eliminate messing around with a .057 decapping pin. The purpose of this test was for my own information, and not meant as a recommendation. As is always the case, your mileage may vary.
  • mongoosesnipe
    Chieftain
    • May 2012
    • 1142

    #2
    The aa/lapua brass uses the ppc spec flash hole I have never had a problem with my redding dies on the lapua brass but I bought a set of rcbs 357 sig dies( also have small flash holes) when I started loading because I didn't want to pay 130 for the dillon carbide dies and omg that was a nightmare decaping pin pulled out every other round, I retuned them the next day and ordered the dillon dies.....
    Punctuation is for the weak....

    Comment

    • bwaites
      Moderator
      • Mar 2011
      • 4445

      #3
      As noted, the PPC case that the Grendel was created from had the slightly smaller flash hole, so its simply a holdover.

      In the past, that size flash hole might have played some part in improved flash efficiency, particlularly if the primers weren't as consistent as they are today. The other issue is that we are extremely unlikely to see an advantage in a gas gun, that has LOTS of other issues that affect accuracy and consistency. A heavy bench gun might, though, and the PPC cases are optimized to that environment.

      This issue is well known, and I thought that all the die manufacturers had gotten on board and only shipped dies with the proper pin, apparently not!

      Comment


      • #4
        I learned that I have just been blowing through the flash holes with my Hornady New Dimension dies I got back in 2009. Never had issues with the pins, but I check my dies and tighten the pins if they are getting loose.

        We addressed this very topic in Volume II of the 6.5 Grendel Reloading Handbook, but not to the extent of checking ES/SD. From this limited test, it does appear that there is not a measurable change.

        The small flash hole could also be for consistent burn across a large temperature range, since Finland commonly sees subzero temps throughout the winter, and is a major consideration when Lapua does their engineering analyses, design work, and testing.

        Thanks for the detailed post with supporting data.

        Comment

        • cory
          Chieftain
          • Jun 2012
          • 2985

          #5
          I've been meaning to do this very experiment for at least a year now. I use the harvey deprimer and the Lapua brass is a pain in the butt. I'll still do the experiment before I take a drill bit to my lapua brass. However, it's going to be great to have this to compare to. Thanks for posting this.
          "Those who sacrifice liberty for security, deserve neither." Benjamin Franklin

          Comment

          • bwaites
            Moderator
            • Mar 2011
            • 4445

            #6
            Why not just use a smaller pin Cory? You can turn yours down in a drill with some emery paper.

            Comment

            • Boostmeister

              #7
              Turning down the pin can also be a PITA. Then you need to turn one down each time you break one. From experience, I can state that the smaller pins are simply not as stout, and you will be turning down a lot of pins.

              As LRRPF52 pointed out, there may be a temperature range over which the smaller flash hole will show a performance improvement. I am also negligent in only having purchased Volume I of the 6.5 Grendel Reloading Manul. As BWAITES surmised, this test was run on a gas gun which may have masked some minor change. I should have indicated in my test data what hardware was used in the test:

              - the firearm is a homebrew AR. 18" Shilen match barrel and bolt, Spikes Upper/Lower, JP roller burnished carrier, Falcon carbon fiber free float handguard, JP captured buffer spring, Giessele 2 stage match trigger, Harris bipod, Magpul PR stock, and Vortex Viper PST 6-24 scope.

              - PACT Professional Chronograph XP, placed 10 feet from the shooting bench. Note: the Oehler 35P should arrive on Tuesday.

              - Environment temp was 64 degrees, dewpoint 57, elevation 1247

              - Load for both the standard size Lapua flash hole and modified brass consisted of 28.3 gr Varget (each charge measured on my RCBS Charge Master), Federal 205M Match Primers, 123 gr Lapua Scenar projectile seated to 2.25" OAL.

              Again, the test was more for my own piece of mind and not meant as a recommendation to open up everyone's flash holes, nor a condemnation of Lapua for the very small flash hole. I had several hundred pieces of Hornady brass, and nearly 300 pieces of Lapua/AA. I set up a simple test to determine (in my gun) if there was any statistically significant difference based on the size of the flash hole diameter. I could have used a larger sample size, run the test with ammo that has been warmed and others chilled to compensate for temperature variables, tried several different powder/primer combinations, and basically driven myself crazy with the variables. I plan to rerun the test again with a larger sample size, using a heavier powder charge, and in some cooler weather (it's changing pretty quickly considering that it is still August here in NW WI). Oh yeah, and I'd better order Volume II of the 6.5 Grendel Reloading Handbook. :^) Thanks for all the feedback guys.

              Comment

              • NugginFutz
                Chieftain
                • Aug 2013
                • 2622

                #8
                While LRRPF52 may have had his dies punching through and enlarging Lapua flash holes with his early Hornady dies, my newer (two year old) Hornady new dimension set came with both regular and slim decapping pins, with the smaller pin installed at the factory. I've never had a single issue with any of the AA/Lapua cases and these dies have given very consistent results. Not a bad option at all, considering their price and quality. On top of all that, replacement pins are easily obtained. (I ordered a couple spares, early on.). Unless you are determined to use bushing style sizing dies, these are an excellent choice.
                If it's true that we are here to help others, then what exactly are the others here for?

                Comment

                • cory
                  Chieftain
                  • Jun 2012
                  • 2985

                  #9
                  Originally posted by bwaites View Post
                  Why not just use a smaller pin Cory? You can turn yours down in a drill with some emery paper.
                  Partly my own curiosity.

                  I honestly expected it to make more of a difference. Not a large one, but noticeable.

                  The harvey deprimer doesn't come with a smaller pin and you can't change out the pins as easy as you can on a die.
                  "Those who sacrifice liberty for security, deserve neither." Benjamin Franklin

                  Comment


                  • #10
                    Originally posted by NugginFutz View Post
                    While LRRPF52 may have had his dies punching through and enlarging Lapua flash holes with his early Hornady dies, my newer (two year old) Hornady new dimension set came with both regular and slim decapping pins, with the smaller pin installed at the factory. I've never had a single issue with any of the AA/Lapua cases and these dies have given very consistent results. Not a bad option at all, considering their price and quality. On top of all that, replacement pins are easily obtained. (I ordered a couple spares, early on.). Unless you are determined to use bushing style sizing dies, these are an excellent choice.
                    Yes, my Hornady dies are from 4 years ago, and everyone that has bought them in the past few years seems to get the smaller pins.

                    Comment

                    • Buster
                      Warrior
                      • Mar 2012
                      • 344

                      #11
                      GREAT findings from Boostmeister

                      I use a Lee Single Stage Press and a Lee Decapping Die and have to bow up on those Lapua cases like a Halloween cat...

                      Comment

                      • Dogue
                        Warrior
                        • Mar 2011
                        • 415

                        #12
                        Weird, I had little trouble enlarging the holes using my Lyman press and Lee Decapping Die. I still have an unopened box of Lapua brass and since I now use the Hornady dies (with the small pin) I'm not sure what I'll do when/if I get around to putting that brass into my rotation. Probably will just enlarge them for the sake of consistancy.
                        Μολὼν λαβέ

                        Comment

                        • Boostmeister
                          Bloodstained
                          • Sep 2014
                          • 63

                          #13
                          After my original posting of the Lapua Flash Hole Test data, LRRPF52 offered:

                          "The small flash hole could also be for consistent burn across a large temperature range, since Finland commonly sees subzero temps throughout the winter, and is a major consideration when Lapua does their engineering analyses, design work, and testing."

                          Great thought! I figured that I had better try to quantify any performance criteria at colder temps. I loaded up a quantity of Lapua standard sized flash hole brass (roughly .058), and a similiar quantity of Lapua brass where I had opened up the flash hole to approximately .078, which is commonly seen in most rifle ammo. I decided to use a more robust charge this time in order to test any effects of the flash hole size. All rounds were loaded with 28.5 gr of IMR 8208 XBR, Lapua Scenar 123 gr bullets, and seated to an OAL of 2.25. To simulate the ammo's use in colder weather, all loaded ammo was cold soaked in a chest freezer for 6 days at -5 F. The ammo was transported to the range (15 minutes drive) in a small cooler, packed with frozen material (Elk & Venison steaks) to keep it cold.

                          Test Conditions: 60F, dewpoint 54, elevation 1247

                          Firearm: homebrew AR with an 18" Shilen match barrel and bolt, Spikes Upper/Lower, JP roller burnished carrier, Falcon carbon fiber free float handguard, JP captured buffer spring, Giessele 2 stage match trigger, Harris bipod, Magpul PR stock, and Vortex Viper PST 6-24 scope.

                          All charges were individually weighed using an RCBS Charge Master, and used Federal 205M match primers.

                          Data was measured using an Oehler 35P chronograph.


                          Control Group: Hornady 123 gr. A-Max, stored at ambient temperature, and fired in sequence.

                          Velocity
                          2454
                          2476
                          2460
                          2478
                          2433

                          Std Dev 18.3
                          ES 45.0
                          Avg 2460.2

                          Testing the "cold" ammo consisted of shooting at six target spots, at 100 yards. The spots were arranged in two vertical rows of three, the left three being for the small or standard Lapua flash hole, and the right target spots for the modified flash hole (large). The rounds were fired in alternating fashion (SM, LG, SM, LG, etc.) such that barrel heating, ammo warming up, etc. were minimized.

                          Std. Lapua Flash Hole
                          2507
                          2497
                          2506
                          2489
                          2489
                          2534
                          2512
                          2531
                          2517
                          2484
                          2494
                          2506
                          2518
                          2498
                          2501

                          Std Dev 14.8
                          ES 50.0
                          Avg 2505.5


                          Modified Flash Hole
                          2525
                          2492
                          2506
                          2509
                          2501
                          2477
                          2484
                          2486
                          2502
                          2485
                          2503
                          2508
                          2505
                          2496
                          2522

                          Std Dev 13.7
                          ES 48.0
                          Avg 2500.1


                          Perhaps a gas gun can mask some minor performance differences. Perhaps I could have tested greater ammo amounts to increase the statistical significance of the testing. I'm sure that there are other variables that could impact the results. Having acknowledged the above, for my gun, in my world, the flash hole size seems to offer no performance difference. I still have a five boxes of AA ammo that once fired, I plan to open the flash holes to .078. I have already performed this on 200 hundred pieces of AA/Lapua brass. I'm certain that there will be some that would say that I have just ruined some otherwise outstanding brass. Again, this test was for my knowledge. This is not a recommendation for everyone to open up the flash hole in their AA/Lapua brass. Your mileage may vary.

                          As a side note. I did not measure the size of the groups, but all were under an inch, and most ran between .5" and .75" with no flyers. I've got 8 lbs of 8208 XBR and will plan to do additional testing. If I haven't put everyone to sleep, just wondering if anyone has ever tried any 4007SSC powder in the 6.5 Grendel? It's just a little slower than CFE223, and I have had good results with it in both my 22-250 and 25-06.

                          Comment

                          • pds
                            Warrior
                            • Dec 2012
                            • 128

                            #14
                            Very interesting test and results. Having shot benchrest for over ten years it was always thought that the small flash hole used in the 6ppc case was just one of the factors that made it so good at shooting such small groups. It would be interesting to run this test again but using a different brand of primer such as CCI450 or Rem 71/2. Thanks for sharing.
                            pds

                            Comment

                            • SG4247
                              Warrior
                              • Aug 2013
                              • 497

                              #15
                              Good info and this matches my analysis as well.

                              I did not try the low temp test however...

                              My tests were focused solely on average five shot group size.

                              Now, I drill all Grendel brass to the same diameter. I want it as consistent as possible.

                              The case volume difference between Lapua and Hornady has a bigger effect on accuracy in my my rifles than flash hole diameter, in my experience.

                              Your results may vary. Thanks for posting your findings!
                              NRA F-Class Mid Range High Master

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