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HUGE BC change with opening up the hollow point.

 
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  #15  
Old 10-14-2007, 09:20 PM
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i don't see what the big deal is. we're talking about a 1.5 MOA difference at 1k. who cares if it's 21 or 22.5, you still have to have it on the right come-up.
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  #16  
Old 10-14-2007, 09:22 PM
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I never thought about it that way. I honestly dont know then. I just know that the difference at 650 yards was 15" which is a huge difference being the velocity is the same.
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  #17  
Old 10-14-2007, 09:24 PM
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Quote:
Originally Posted by davewilson View Post
i don't see what the big deal is. we're talking about a 1.5 MOA difference at 1k. who cares if it's 21 or 22.5, you still have to have it on the right come-up.
That was the difference for someone else's drops and at 1K to boot. My drops at 650 yards were 15" of difference and out of my 308 I dont have enough velocity to make 1K with a .451 BC where as nearly .580 will more than make it. It doesnt seem like a big deal but for the limited 308, it is a huge deal.
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  #18  
Old 10-14-2007, 10:05 PM
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Fellas,
As someone who has been trimming meplats sinse 2001 when Dave Tooley made his first prototype trimmer for my 338, the MOA drops you are seeing are NOT from the meplat.
The issue I see here is that some shooters here are intepreting the trim meplat = massive loss of BC and that simply isn't the case.
We have a Oehler M43 with 300yds of coax cable to measure differences in BC and I've never seen that big of a difference when trimmimg the meplat only. Several years ago we did some testing for a bullet maker across that same M43 and we shot 100s of rounds in one day using everything from 6mm up to 338 caliber and all sorts of bullets. With tips, without tips and trimmed, trimmed only, not trimmed at all, A-Maxes etc etc etc. We never saw that amount of BC loss ever.
MikeCr is correct in that opening up the hole in the meplat "shouldn't" effect the aero that much either if my research is correct. But obviously something is happening. I use an tip made for my dremel tool to debur that hole inside the meplat, but I have never opened up the hollow point to help with expansion like you guys are talking about. I would be interested in hearing exactly how you open up that holow point. How the bullet is held, how it's drilled how deep, etc etc??
What I suspect is that how you are opening up the hollow point may not be centered causing a dynamic yaw in the bullet which in turn causes a bigger drop in MOA. But I haven't heard anyone mention that their groups opened up that much in the groups that dropped a lot more. So I'm not convinced that is the case either. But something is happening.

Just don't claim that trimming the melpat .005" is causing this or that there is no increase in accuracy in doing it either. I've done to many to know it's not the case. In regard to accuracy gain of just trimming... beleive me I've heard it all. But I've also seen it measured repeatidly in test conditions to know that the BC variation is improved on trimmed bullets. Not much but the numbers are there to support it. So even though you may think you can't see it on paper it is there. Remember the distanses we shoot here gentleman. The condition changes at these ranges mask a lot of small things. Don't over estimate that. I wish it was as simple as shooting a group without trimming the meplat and another group right behind it with trimmed bullets and the second group was 50% smaller. But it isn't that simple with the equipment we have available to us these days. Let's face it the groups that we shoot at long range are awesome. There is no more "big" improvements to make. So if it can be measured and the numbers show an improvement, it is there even if you can't readily "see" it.
Just like at Indy. It use to be what speed barrier are they going to reach the next year during time trials. Then when they went over 200mph barrier it slowed down to only a couple mph difference per year until it plateaued around 230mph before the big rules/organizational changes.
Same with our guns, we have come a long ways in the last 10-15 yrs in long range equipment and it's good stuff. Small changes are not going to be glaringly obvious on the target anymore. We have reached into the area of statisical control with smaller sample sizes.

hope this helps. I would definetely like to know exactly how you are drilling out the hollow points though. That is where this drop is coming from. Just a matter of figuring out why.
Just like the ones in BR who are pointing thier bullets with a secondary point die and closing up the meplat. Some have reported worse accuracy, some reported more drop, some reported less drop. It was found that when bumping the nose to close up the meplat was squashing the ogive of the bullet and causing a lot of issues. So too much of a good thing. But right now the jury is still out as to whether pointed meplats are more accurate than trimmed meplats. Another experiement for another day.

Steve
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  #19  
Old 10-23-2007, 07:40 PM
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Steve,
Very interesting comments. All I can say is thank god for A-Max's and roll on Richard and his alloy tips! Please, some 200gn AT 7mm's soon!!!!!!
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  #20  
Old 10-23-2007, 09:40 PM
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i've been experimenting with opening up the hollow points to help insure expansion. and my technique is simply holding on to the bullet in one hand and the drill bit in the other. now i know everyone is saying you can't get the "new" hole exactly in the middle, concentricity issues,and all i can say to that is BFD. my reasoning is simply look at the tip of an unaltered SMK bullet. most of them do not have a nice looking tip on them. one side of the metal of the hollow point is as much as .020 higher than the other side. talk about out of balance.i start by using a .043 drill and then run a .047. it doesn't cut very deep maybe .06 or so, then it's hollow until about 3/8 down and you hit the lead.unfortunately, this makes nasty burrs at the tip of the hollow point and i use mother's nail file to clean em up.when i'm done, my tips are much more uniform than from the factory and a 047 hollow point to boot. i've shot these at 500,600,800, and 1k so far and i can't determine any difference in accuracy.interestingly i can't see any difference in POI either.the last 3 outings i loaded three unaltered bullets for just this purpose and they hit at basically the same height. to be honest i don't really care if it would affect the BC as long as it makes them expand every time, i think that's more important.
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  #21  
Old 10-24-2007, 03:55 PM
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Quote:
Originally Posted by davewilson View Post
to be honest i don't really care if it would affect the BC as long as it makes them expand every time, i think that's more important.

I feel exactly the same way. My only issue is starting out with a MV of 2567, it doest take long for a bullet with 20+% of it's BC loss to go transonic and tumble. That really limits my weapons range. If I had more starting velocity, than it would be of less concern to me having such a BC loss, other than I might as well use a hunting bullet as opposed to a SMK that takes alot of time to uniform and drill out. I loose time and gain expansion. I can shoot an ACCUBOND with a better BC than the modified SMK and I dont loose any time and still have expansion. The reason it sucks is because with an unmodified SMK 190 in my rifle, it is long range shooters dream, it just doesnt open up. Modifying it will make it open up but then I loose the qualities that make it the "perfect" long range load.

I am glad you didnt loose any of your BC. Unfortunatly, I am not as blessed.

My solution for my next hunt (sitka blacktails next week) is simple. I am taking my 300 RUM instead of my 308.
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Long range shooting is a process that ends with a result. Once you start to focus on the result (where the shot goes, how big the group is, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
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