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RPR 338 Lapua

This is a discussion on RPR 338 Lapua within the Ruger Bolt Action forums, part of the Rifle & Shotgun Forum category; Good morning. Does anyone know the receiver, barrel shank, and locking lug dimensions on the RPR that's made for the 338 Lapua? TIA....


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Old December 6th, 2019, 05:17 AM   #1
 
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RPR 338 Lapua

Good morning. Does anyone know the receiver, barrel shank, and locking lug dimensions on the RPR that's made for the 338 Lapua? TIA.



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Old December 15th, 2019, 03:10 AM   #2
 
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Mine got delivered a couple days ago and I measured everything myself so now I know. It is much beefier than my Savages of the same caliber. I look forward to shooting it. Always been a Ruger and Savage fan but this thing has a bigger receiver, barrel shank, and more locking lug area than the Savage, so it should be able to handle the bolt thrust much better.
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Old December 15th, 2019, 03:43 AM   #3
 
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Old December 16th, 2019, 05:11 AM   #4
 
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What are the locking lug dimensions. I would like to compare them against the 300 PRC I just received. They are probably the same, as it is also built on the big action.
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Old December 16th, 2019, 10:30 AM   #5
 
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I will have to look at my notes when I get back home as I don't remember off the top of my head. The lugs are tapered slightly from bottom to top so the top is more narrow. It's also slightly rounded at the top so I tried to get measurements as close as I could with my caliper at the bottom and top of the lug. All of the magnums should be the same size. That's my understanding anyway. It is a beefy action though. The receiver was something like 1.412" wide, and the barrel shank in front of the nut was about 1.211". I was pretty impressed at how much larger it is than a Savage or Remington action, but Ruger has always been known for building a very strong action/lockup.
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Old December 16th, 2019, 10:32 AM   #6
 
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So bear in mind, with the taper and rounded corners, my measurements may not be exactly right, but should be close enough to give a good idea.
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Old December 18th, 2019, 05:04 AM   #7
 
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My measurements I got with my caliper as best I could get them with the lug being tapered, were .404" wide at the base, .344" wide at the top, .173" high and .461" thick. I snapped a quick pic with my phone last night of my measurements and turns out it didn't focus so the photo is a little blurry. But I believe that's the numbers as best I could make them out.
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Old December 18th, 2019, 06:47 AM   #8
 
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They are the same.
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Old December 18th, 2019, 08:38 AM   #9
 
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I figured they were all built on the same size action. According to my measurements that's about a 30% increase in surface area compared to the lugs on my Savage of the same caliber so it should handle the bolt thrust a lot better. Savage increased the thickness front to back of the lugs for better shear strength, but they're the same width and height as their standard action, so the displacement area is no bigger.
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Old December 18th, 2019, 04:16 PM   #10
 
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The bolt diameter right in front of the handle is .989, the diameter of the 3 locking lugs is .983. You have to measure using one of these. (I used to be a tool maker when I was a kid and have a lot of tools left). These let you measure an odd number of positions, or measure out-of-roundness.

Why there is .006 difference, only Ruger would know, unless it is simply for working clearance.
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Old December 19th, 2019, 05:17 AM   #11
 
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That's a cool tool indeed for that purpose. I may have to get my hands on one of those. However, that's not what I was measuring and doesn't tell me the surface area that engages in the lug abutments. That I was able to measure with a simple caliper, measuring the width and height of the lugs. The diamater all the way across really doesn't tell how much is engaged in the recesses. I believe to do that you have to measure the surface area of the rear of the lug where it engages the abutment in the receiver. I think I got the measurement pretty close, at least for my purpose of comparing it to a Savage bolt any way.
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Old December 19th, 2019, 05:26 AM   #12
 
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However, that's not what I was measuring and doesn't tell me the surface area that engages in the lug abutments.
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Old December 19th, 2019, 05:40 AM   #13
 
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Quote:
However, that's not what I was measuring and doesn't tell me the surface area that engages in the lug abutments.
I understood that. I was adding the measurements to those that you already had posted so that the complete geometry was noted. In order to truly calculate surface area you need all the measurements, including the diameter. Once upon a time, I could calculate it out, however it has been 40 years since I have had to do geometric calculations to that degree.

In order to truly calculate the engagement surface (which is the back of the lug, you would also need the inside diameter that the bolt passes through before rotating into battery. Then using the OD of the bolt and the ID of the bolt mortice, you could work out pretty close to what it is by figure out the engagement area of the donut circle cut into equal slices of x degrees of rotation. It is doable, but you have to also figure in all the radii that there is on the bolt lugs as you have truly noted.

The major diameter of the lug cutout doesn't come into the equation as it is bigger than the OD of the lugs.

There are tools that accurately will measure the lugs at the base, etc, but they are hideously expensive.

At one point in my early 20's, I ground roller dies for making automotive pulleys. The measurements were taken at a distance down the "V" so a precise depth was set, then you measured. 40 some years ago, that handheld tool cost 1000's.

You have to watch eBay close. Do NOT buy the first one you see, unless it is a really good price.

This one is way overpriced. It does have a best offer option though. Offer them $50 or $60 as that is in the range that I bought mine for. There is a diminished pool of people that are buying machinist tools, especially specialized ones.

https://www.ebay.com/itm/Mitutoyo-V-...kAAOSw-TFd7yH4

This is also a very useful tool to have. You can measure LOTS with and without the attachments. This is just an example.

I chose this one to show you because it has the different anvils, so if you ever decided to look for one, you would know what goes with it.

I have 2 of these and use them a lot.

https://www.ebay.com/itm/Mitutoyo-0-...gAAOSwAHtZ5ifb

I bought the tri-anvil mic because I shoot BPCR rifles and cast my own bullets. I like to check to make sure that I am casting round ones. (at least as good as a casting can be).

Last edited by drcook; December 19th, 2019 at 05:53 AM.
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Old December 19th, 2019, 07:34 AM   #14
 
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Appreciate the additional info and always eager to learn. I believe I understand most of what you're saying, although a lot of it appears over my pay grade. The inside diameter you referred to, if I understand correctly, is where the abutments are cut in the receiver and the small neck between the lugs and main body of the bolt would rotate. I measured the height of the lugs just in front of that. Assuming (and we know what that does) that the inner ring as you put it is the same size as the neck on the bolt (likely slightly larger to allow clearance for bolt rotation), I thought that would indicate the lug abutments close to the same size as the lugs themselves. Again, much of this is above my pay grade. However, I think the way I did it, and then measured in the same fashion my Savage bolt lugs, still gave me a pretty good reference that show the Ruger bolt has more surface area and as a result is a stronger lock up than the Savage bolt for such a cartridge size. I myself haven't had any trouble with my Savages yet, but have heard that the lugs don't displace the bolt thrust enough and some have commented on experiencing lug setback as a result. So I was comparing the two bolts to see if the Ruger had enough size to it to better handle the thrust imposed on it.
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Old December 19th, 2019, 10:35 AM   #15
 
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I understand exactly why and what you were doing. I am just trying to help by giving you the measurements that I have the tools to do so. There is not much metal there when you really look at it, and bolt set back is not a good thing. Either the metal is compressing or moving. Moving metal is not good.
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