Ultimate Suppressed Subsonic 338BR Whisper Rifle
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By Mike Semanoff

For years I was of the belief that working with non-standard or wildcat cartridges was a waste of time. With so many great high performance cartridges on the market, why would anyone want to deal with the inconveniences of fully custom rounds? The increase in performance is hardly worth the hassle of limited commercially available ammo and having to create custom reamers and dies. That is, until I went down the rabbit hole of shooting subsonic .338 rounds through my Thunderbeast 338BA suppressor

At the time, commercially available subsonic .338 didn’t exist or at least from any sort of mainstream source. I considered loading subsonic .338 Federal, but didn’t like the large case capacity. I thought about 338 Spectre, but didn’t like the non standard bolt face. I considered a whole bunch of wildcat options, but finally settled on the 338BR which is also known as 338 Whisper.  I like that it has a standard .308 bolt face, small case capacity, and brass can be easily formed from available 7mm BR brass.

Ultimate Suppressed Subsonic 338BR Whisper Rifle

With the cartridge selection complete, the next action item was barrel selection and twist rate. To determine a suitable twist rate I used the stability calculator on the Berger Bullets website. This tool lets you enter the bullet parameters, velocity, and twist rate to come up with a range of stability. That gives you a pretty good idea of where you want to be.

I went with 1:6.5 and consulted a few friends in the industry before finally ordering from K&P Barrels.

With the barrel ordered, I was fully committed to the project and I ordered reloading dies from CH4D, had a custom chamber reamer made by Pacific Tool & Gauge, and picked up a good supply of 7mm BR brass from Peterson Cartridge.

The rifle was built by Alamo Precision Rifles down around Hurst, Texas and it’s powered by the Defiance Machine Ruckus action. This is my first rifle built on a Defiance action and I couldn’t be happier.

Ultimate Suppressed 338BR Whisper Rifle

One thing I worried about was reliable feeding of the odd-shaped cartridge, but this action had no problems. I found that I had to be careful loading 10-round mags for reliable feeding but the 5-round mags did great. It was more of a magazine geometry issue than anything to do with the action. 

Ultimate Suppressed 338BR Whisper Rifle

The barreled action has a Timney Triggers Calvin Elite trigger and sits in a bedded Grayboe Renegade stock. The Renegade is a solid fiberglass/epoxy stock with a classic McMillan A5 profile. This stock had flush cups installed by Alamo Precision before they sent it off for paint.

Ultimate Suppressed 338BR Whisper Rifle

The build is topped with a US Optics FDN 17 which gives me a little over 90 MOA of internal adjustment with my 2O MOA base and an additional 60 MOA in the reticle. 

With the rifle complete, it was time for some load development which required the help of QuickLoad and some input from a few experienced friends. After trying several powders I’ve used for subsonic 300BLK, I ended up with VihtaFouri N32C (TinStar) for full case capacity and full powder burn. Other options like Reloader 7 or AA 1680 didn’t give me a case fill that I felt good about and ended up with mixed results. 

Ultimate Suppressed Subsonic 338BR Whisper Rifle

What I finally ended up with is a 300 grain Lapua Scenar going 1060fps with an SD of 8 over a 15-shot string. Once I dialed in seating depth, I was able to get several 5-shot groups between .75 and 1 inch. Not what I was expecting, but apparently that’s pretty decent for subsonic reloads.

Ultimate Suppressed Subsonic 338BR Whisper Rifle
.308 Winchester (left), .338BR (right)

With load development complete, it was off to the range to stretch the legs and see how the calculations hold up. I had a couple range sessions out to 300 yards and had no problems connecting.

Ultimate Suppressed Subsonic 338BR Whisper Rifle

The first time out with a camera I went for 400 yards and was about 20 MOA low. I adjusted and took some direction from my spotter and was able to connect after a few shots.

I couldn’t understand why the calculation was so far off and began playing with my inputs.  The wind picked up quite a bit and I didn’t have time to trouble shoot before heading home. 

I ended up trueing the ballistic coefficient rather than the velocity because I had the published supersonic BC entered in the calculator. Adjusting that input and switching to the Shooter ballistic app seemed to work and we headed back out for another session out to 500 yards.

Ultimate Suppressed Subsonic 338BR Whisper Rifle

After a quick zero confirmation at 100 yards, we went right out to 500 and dialed it right in. It took two shots to get on and then went three in a row. The goal was 500 yards and time didn’t let us to take it any further, but we will at some point.

For those who want the numbers, it took 64.5 MOA to get out to 500 yards with a flight time of 1.5 second. Velocity on target is calculated at 941fps and 590ft-lbs of energy for the 300gr Lapua Scenar. 

Ultimate Suppressed Subsonic 338BR Whisper Rifle

My next project is to see how far we can take it. If I max out the scope adjustment I should be able to hit 650 and if I use the reticle for the rest then 950 should be within reach.

Ultimate Suppressed Subsonic 338BR Whisper Rifle

Strapping on a Charlie TARAC or a Nightforce Wedge Prism will let me go even further. With all the tools available, I know 1000 yards is absolutely possible with an approximately 160 MOA of needed elevation. 

Ultimate Suppressed Subsonic 338BR Whisper Rifle

Overall this has been a successful project that I’m glad I invested the time and energy. A lot of people ask me what’s the purpose of the build. I don’t know if there is a practical purpose, other than maybe some quiet hog hunting. I do know that I learned a lot and everyone who shoots it smiles and gets a good laugh. 

This was a really fun project to see though and if all I ended up with is a quiet rifle that makes people laugh, then that’s good enough for me.

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  1. I don’t see myself building one of these.


    These are the kinds of articles / stories I’d like to read more of on TTAG. Fun to read about someone else’s dream gun and them making it happen.

  2. Just because you can doesn’t mean you should……………but this looks like it was an extremely fun build with a lot learned in the process. I may never work with subsonic rifles (suppressor no go at state level) past .22lr but always good to learn some new ideas.

  3. Mom: we have 8.6BLK at home!

    On a more serious note, you did what they haven’t finished yet and made it work. That’s super cool.

    • Was a ton of work. I looked at the 8.6 at Shot and it seem to have more case capacity. Wish this BR had a little less to get better case fill with more powder options.

  4. Ya know, that is a really nice looking rifle. Nice clean lines, not gimmicky, has a purposeful elegance look to it.

    Whats that bipod – brand?

  5. A while back, I was wondering about possible tactics in a situation where an enemy occupies our nation. Obviously, having the ability to take out enemy combatants at 800 yards is highly desirable in such a situation. While there are countless rifles and calibers that hit targets at 800 yards with boring regularity, all of them send bullets at supersonic speeds which have significant sonic signatures. That significant sonic signature means your enemy will know WHY one of their comrades just keeled over. And your enemy will also have a pretty good idea WHERE the shot came from as well as how far away the shooter was. That significantly increases their odds of capturing the shooter.

    I concluded that a rifle and ammunition platform that could send subsonic bullets out to 800 yards with boring regularity was highly desirable. With such a platform, an enemy combatant would keel over and none of his comrades would know why. (Bonus: that may also enable a second shot to take out a second combatant.) Even more important, his comrades–upon realizing that a bullet just killed their comrade–would have no idea where the shot came from or how far away it was. That significantly reduces their odds of capturing the shooter.

    It looks like this is exactly such a rifle and ammunition combination. I sure wish I could afford to make that rifle and ammunition.

    • A Mongolian archer hit a target 556 yards away.
      Cant remember his name but ghengis khan had somebody scratch about it on a rock.

        • Rider/Shooter,

          Hitting a human size target at 50 yards with archery is pretty easy.

          Hitting a human size target at 100 yards with archery is very difficult.

          Hitting a human size target at 200 yards and beyond with archery is nearly impossible.

          I would much prefer a rifle that reliably delivers a subsonic bullet on a human size target out to 800 yards.

        • Uncommon; I said I wish I was more skilled with it, which is not to say I’m only passing familiar with it or had no basic skills. I’ve ruined more than a couple arrows because another one was in the way…

  6. Cool build and a fun project! I can’t see myself investing that much in a giggles gun, but I have put a lot of sweat equity into similar fun projects.

    My favorite was to build a .308 that would shoot every bit as well as my buddy’s spending H&S Precision, for under $600. It took a while to collect the parts and pieces, but it was based on a 98 Mauser action, surplus Parker-Hale British army competition barrel and trigger group from SARCO, and a Riflestocks.com thumbhole stock. Shot about 3/4 MOA consistently with handloads. After I build it I had accomplished my goal, and sold it to another friend. Now I build my own scary ghost guns on the mill and lathe! [for personal use only, obviously]

    • I think if the sound signature from this type of rifle is low enough then it would be far from a giggles gun and instead be yet another dozen pounds of exceedingly useful gear you would very much wish to add to yer shtf loadout.

      • Oh c’mon, awaiting moderation? Again? Still? So what word/s triggered the triggering this time; was it: sound signature? Or rifle? Or guns? Or shtf? Or loadout? Or maybe it was giggles?? I think maybe the moderator might need a little moderating.

  7. 500 yards at subsonic speed is pretty awesome. I’ve made plenty of hits at 200-300 with 308 subs and it’s funny how long the time from pfft to ping feels. 500 would probably feel like shooting a grand with a supersonic round. Did… I… Oh, target moved. Wait… Ping!

    • You-know-who is a pedo,

      500 yards at subsonic speed is pretty awesome.

      Exactly what I was thinking!

  8. And now for the really fun question: are bullets available with those incredibly high ballistic coefficients which also expand to a 3/4-inch at impact speeds between 800 and 1000 feet-per-second?

    Follow-up question: what sub-sonic ballistic coefficient did you determine that those bullets have?

    • I figured it to be about .400. That got the calculations to line up with reality when I had a 1060fps muzzle velocity.

      • Hmm. Seems like such a heavy-for-caliber bullet would have a much higher ballistic coefficient. Then again I am thinking in terms of supersonic flight ballistic coefficients.

        • That’s what got me as well. I was calculating with the published supersonic BC and wondered why things weren’t lining up. I ended up truing the BC rather then velocity and that seems to work in this case.

    • Maker 300gr .338 monolithic copper hollow points with petal cuts will open up down to at least 850fps. I have never tried them slower because I never shoot that far so I never gel tested at a lower velocity. They have a lower BC though (I was figuring .41 but Mike’s experience here shows that I overestimated significantly) and at least with me shooting them from my gun aren’t as accurate. I only intended my .338 Specter AR-15 to be a 200 yard gun. My biggest problem with these long (1.75″) Makers is that my chrono errors on almost every shot. I gotta find somebody nearby with a Labradar or something.

  9. Very nice rig. Now you’ve got this poor-boy deplorable wondering about a subsonic 405gr load with scope n suppressor for my Marlin 1895.

  10. I’m doing an AR-10 build with a .375 Raptor, so your story about doing sub-sonic is very compelling. Dang, I’d have to get a different barrel for it now…and build another upper too. 😀

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