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Thread: New RVA for my AR

  1. #1
    Senior Member sholo's Avatar
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    New RVA for my AR

    I've been wanting to make an RVA for my AR2078 for some time now, but not having a metal lathe or milling machine, I was a little hesitant about how to go about it using only a cheap Mastercraft drill press, Dremel, etc... I'm sure (?) I could have whittled something out - maybe - but I AM sure it would have been fairly crude with brutal tolerances!

    So I contacted none other than our very own SeanMP to see if he could possibly make me one, or at least give me some pointers on how to attack the project using my "caveman" tools. Man...am I ever glad I did! I was told to bring all my factory pieces down and he would whip one up for me on the spot...

    Here is the finished product with the factory one above for comparison:
    m_AR RVA 001.jpg

    I went with this style as I wanted both an RVA, but still have the gun retain its factory look. The new cocking block is .200" longer than stock to allow a bit more adjustment range, and now sits right tight to the new end cap. Right behind the cocking pin a hole was drilled/tapped 8-32, where a nylon plug, the smallest spring I've ever seen, and an 8-32 set screw make up a "brake" to keep adjuster/spring guide from moving once set where I want it. The threaded end of spring guide was also drilled/tapped 8-32 for a set screw (with red Loctite applied), and spring tension is adjusted by inserting an allen key through a small hole drilled in end cap.

    Seeing as the end cap now sits tight to cocking block, it was bored .312" x .450" deep to allow adjuster enough room to be back off. There is a shoulder milled into the cocking block to prevent it from backing completely out:
    m_AR RVA 002.jpg

    When all was said and done, I ended up with just over 1/2" of total adjustment range, which should be more than enough to tune the gun to whatever velocity I desire:
    m_AR RVA 004.jpgm_AR RVA 003.jpg

    Thanks a million, Sean! Not just for the great RVA, but for you and your wife (and MacKenzie) allowing me into your home, the great hospitality, patiently answering my numerous questions, and showing/explaining the basics of working with a lathe/milling machine. I learned more in an afternoon watching you than I have in the last year reading "how-to" articles. It was a most enjoyable afternoon, for sure (for me, anyways!). Only downside is I now have to convince my wife that I NEED those same tools. Maybe I'll have to get her that corner tub afterall...

    Cheers,
    Todd
    Todd

    Don't sweat the petty things, and don't pet the sweatty things!

  2. #2
    Senior Member SeanMP's Avatar
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    Go with the corner tub for sure.....that's good for a lathe. Throw in some marble and ya probably got a mill

    As Sholo said we had a great afternoon zinging out some parts. And it wasn't all one sided as he saved me from slipping deeper into my personal nightmare...AKA "Princess Sheridan" .....more on that later

    My only regret is that we didn't have time to make sure everything was going to work together....as designed.

    You just need to get another full length spring to see what the velocity range is going to be.
    Last edited by SeanMP; Feb 04 2013 at 09:00 PM.
    Sean

  3. #3
    Senior Member sholo's Avatar
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    Well...I just couldn't wait for a new spring to test out my new RVA, so I semi-assembled gun (no stock or scope), filled the 13 ci bottle to 2500 psi, and took some shots over the chrony to get an idea on what sort of adjustment range I could get out of it. Numbers were a bit surprising, to me anyways.

    *NOTE* - Test was done using my clipped hammer spring, which IIRC, is about 5/16" shorter than a stock spring.
    AR2078 in .177, HPA regulated at 1500 psi (2500 psi fill), 10.34 gr JSB Heavies, 5-shot avg:

    With the RVA backed all the way off, the gun wouldn't fire - valve was "locked up" for the first 4 full turns in! Not near enough spring to open valve @ 1500 psi. New spring should remedy that though, allowing me a couple more turns of adjustment. Alternately, I could lower output pressure to 1100 or 1200 psi, but the gun shoots very well at 1500 psi and is really quiet when tuned to around 720 fps (which is my 12 fpe limit for WFTF using 10.34 gr JSB's).

    The following numbers are at 1 turn in (more preload) increments:
    5 - 179.2
    6 - 357.1
    7 - 587.9 (this is where "stock" would be using factory cocking block - i.e: spring resting flush with end of block, neutral position, if you will)
    8 - 731.4
    9 - 853.5
    9.5-870.7

    At 10 turns in, the bolt was about 1/8" short of being able to lock into place. Not sure if the spring was at coil bind, or the guide was bottoming out in hammer Any thoughts?? When I have it apart to change spring, I might (?) shorten the guide by 3/16"- 1/4" to see if that gains me anything (just don't tell Sean!). I'll try the new spring first though, and if it doesn't bind until the 10th turn in, I will know for sure if it's the guide or not. I suppose it really doesn't matter if I get that last 1/2 turn of adjustment, as even at 9 turns in it is quite loud, and at 9.5 turns it's even louder with only a 17 fps gain, so all I'm really doing is hurting my efficiency.

    I realize these numbers mean diddly-squat seeing as I'm changing springs anyways, but it settled my curiosity for now and gives me an idea on what to expect. Once I get the new spring installed, I will do a proper set of shot strings and shot count at various settings and post my results.


    Cheers for now,
    Todd
    Todd

    Don't sweat the petty things, and don't pet the sweatty things!

  4. #4
    Moderator rsterne's Avatar
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    Assemble the gun without the hammer spring.... you will quickly find out if the guide is binding at 10T in.... If the spring is going to coil bind, it will do that even sooner with a spring that hasn't been clipped....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  5. #5
    Senior Member SeanMP's Avatar
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    Todd any progress?

    I was just fiddling with some parts here and noticed that the skirt on the hammer contacted the cocking block long before the spring came to coil bind.
    Sean

  6. #6
    Senior Member sholo's Avatar
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    Quote Originally Posted by SeanMP View Post
    Todd any progress?
    Not really, been busy with life's little hassles lately

    I did as Bob suggested and took the spring out and tried it again, and it still "bottomed out" at 10 turns in.

    Not a big deal though. I think I'll just leave it as is as I am able to adjust from 179 fps - 870 fps (.74 fpe - 17.4 fpe) using a clipped spring. If my thought process is right (big "if") with a full length spring I should gain a bit more at the top end (at the cost of efficiency/shot count), and maybe be able to utilize a couple more turns on the lower end which was unavailable with the shorter spring (valve lock).

    Seeing as this is my primary target rifle, I couldn't be more happy with the preliminary results. I can now shoot sub-500fps (postal matches), 12 fpe (WFTF), or 20 fpe (Open PCP FT) with the same gun. Just a few turns of the RVA and I'm good to go!

    I'll be getting another spring on my next order with Eric, and will (to the best of my ability) do some shot strings/shot count testing at various settings for you all to analyze!


    Thanks again,
    Todd
    Todd

    Don't sweat the petty things, and don't pet the sweatty things!

  7. #7
    Senior Member sholo's Avatar
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    Damn...."timed-out" trying to edit my post!

    I suppose if I absolutely wanted "all-I-can-get" I could just remove some/all of the skirt and gain a turn or two, but I have a feeling once the longer spring is installed I may (?) already be at the point of diminishing returns. Even with the clipped spring, I gained an average of 88 fps for every 1/2 turn in up to turn 9, but from 9 - 9.5 turns I only gained 17 fps and LOUD!!! Pretty sure I'm already wasting air...unless I start using 16 gr EunJins (and I do have some ) which could make use of the last turn or so, but we'll see.


    Todd
    Last edited by sholo; Feb 20 2013 at 09:14 AM.
    Todd

    Don't sweat the petty things, and don't pet the sweatty things!

  8. #8
    Senior Member sholo's Avatar
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    I still haven't picked up my full length spring yet, but I went ahead and did a shot count/efficiency test using my clipped spring anyways. After looking at the results, I'm now thinking that i might not even bother replacing spring...

    Not sure if this is the right method, but this is how I did the shot count portion of test: I didn't want to fill to 3000 psi and shoot down to 1500 psi for each RVA setting (especially at the lower settings), so I pumped bottle to 2750 psi, then counted how many shots I got for every 250 psi used at turns 7 - 9.5, then multiply that number by 6 which would should roughly translate to a full 3000 psi fill and shooting down to the 1500 psi set point. Turns 5 and 6 I only used 100 psi (multiplied by 15) as I didn't think I would have enough pellets to finish!

    We all know that the gauge on 13 ci bottles are not the most precise as far as the markings go (and I wanted to be as precise as possible), so I took factory gauge off and mounted my 3500 psi gauge that I use to check regulator output as it is marked in 50 psi increments as opposed to 500 psi on factory one. Looks goofy, but it worked!!!
    m_AR reg set up 004.jpgm_AR reg set up 005.jpg

    I started from 5 turns in on the RVA (from minimum preload setting) as turns 1-4 the valve was locked up.

    AR2078 / 13 ci Bottle reg'd @ 1500 psi / 10.34 gr JSB Heavy / Clipped H-Spring

    RVA____Velocity____FPE___#Shots___PSI/Shot_____Efficiency fpe/ci

    5_______179.2______.74______780_______1.9_________ 0.42 (? doesn't seem right ?)

    6_______357.1_____2.93______510________3__________ 1.11

    7_______587.9_____7.94______276________5__________ 1.62

    8_______731.4____12.29______180________8__________ 1.65

    9_______853.5____16.73______108_______14__________ 1.34

    9.5_____870.7____17.41_______66_______23__________ 0.85

    Even with the clipped spring I pretty much already have the adjustment range I'm looking for. A longer, full length spring may give me a little more fps/fpe, but the efficiency is already taking a nose-dive as it is, so unless I'm missing something (?), more preload won't really gain me anything.

    Thoughts....?



    Todd
    Todd

    Don't sweat the petty things, and don't pet the sweatty things!

  9. #9
    Moderator rsterne's Avatar
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    I would agree, you have the range covered.... around 9 turns is where you want to be.... or less if you want less power, more shots.... The very low efficiency when all the way out is typical.... simply no energy in each shot....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  10. #10
    Senior Member SeanMP's Avatar
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    I agree as well Todd. You've got the full range covered and your desired range (12fpe) is right in the midrange. Also looks like your highest efficiency point as well...Bonus!

    Your laughing in that caliber
    Sean

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