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Posted: 6/7/2026 8:46:59 AM EDT
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Hoping someone here can answer a couple questions for me. Other than the feed ramp being modded for center feeding, is a 450bm and 458sc barrel extension a standard AR barrel extension? Are the bolt lug sizes, shapes, etc still all the same? Basically, can you use a standard AR15 barrel extension (with a slightly modded feed ramp)? I can not find a concrete answer to this anywhere. Next question. Can anyone with one of the above, test fit a 45 ACP in their bolt and see if it fits? Google says 450bm and 458sc use a .473 rim diameter. 45acp is .480. I am curious if one of these bolts could be opened up for use with 45acp. I measured all the different 45 brass I have and the largest measured .476. So I feel like it could easily be opened up a few thousandths and fit. What is my reasoning for asking this? Well I am building a couple 3D printed pump action AR's and I want one to be in 45 (for suppressed use). I am wanting to take a blowback barrel, thread the breech end and add a standard barrel extension for a locked breech. I have done exactly that for 9mm already, except I started with a barrel blank. But I was able to take a standard 223 bolt and opened up the breech face on my lathe to fit 9mm with no issues. So I was curious if I could do the same with 45 using a 450bm bolt. I'm thinking yes. |
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Other than feed ramp geometry changes, they are pretty much the same, but they are NOT standardized. That said, the nominal lug measurements, threading, length, pin position, etc. (pre ramp cutting) should all be in the same spec range as mil-spec. Unloaded, any AR-15 bolt using nominal mil-spec geometry (i.e. not PCC blowback BCGs) should properly close on any AR-15 lug, as the requisite geometries are nominally the same. Again, the modifications to the feed ramps for large bores vary by manufacturer. There is no standard for large bore extensions. Just because a manufacturer uses a central ramp doesn't mean their cuts, grinding, and polishing on feed ramps is the same as another manufacturer for the same caliber. To go beyond .450BM and .458S, some .350 Legend barrel manufacturers sell them with a single large bore ramp, too. Others use widened side feed ramps. Others go with standard M4. I'm a big fan of checking and, often, widening and polishing feed ramps for most any barrel >= .30 cal to ensure smooth feeding from a broad range of magazines and ammunition weights. |
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Originally Posted By homebaker: I don't see why it wouldnt work. I have used these in the past with good results. https://triuneprecision.com/product/ar15-big-bore-barrel-extension/ Yea I have used one of those on a 9x23 build and it works great. Machine work was top notch. I wanted to order a couple more, but they have been out of stock of them for months and do not respond to emails. I am starting to wonder if they are still in business. |
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For all of our builds, we use standard mil-spec extensions. For any of the cartridges over .257 caliber, we modify the feed ramps to make a large, central version. While not truly standardized, we try to 'blend' the center ramp with the existing ramps, to not remove too much material. For the bolt, 45 ACP should work in these, most of the bolts are opened up slightly more than rim size, to prevent dings and such on the rims from preventing feeding / seating. 45 ACP has been chambered in DI guns some 20 years ago - Ron Williams is the guy who made that happen. |
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"You got out once. You dip so much as a pinky back into this (ARFCOM) pond... you may well find something reaches out... and drags you back into its depths".
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While Google says that the 45acp has a rim diameter .480”, all the reloading shell holders and plates, use the same shell holders as the 30/06 based cartridges. Which is .473”. SAAMI spec for the 45acp is given as .480-.010”. I’m will to bet that a 45acp case will fit into a 450BM bolt. One can actually think of the 450BM as nothing but a 45acp Super Duper Magnum.😁 |
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Keep in mind, the rim/groove may be different on the 45 ACP which could cause the extractor to rim bind. The rim/groove is not the same between the 450 and 458. 458 extractors will work for 450, but many times 450 extractors are not cut correctly to work with the 458 SOCOM case. You can test your extractor with a 45 ACP case, by inserting the rim in the extractor hook, pull forward on it, then roll the case out over the nose of the extractor. There should be no binding at all. If it hangs on the extractor hook, you're going to have ejection issues. Tony Rumore Tromix You don't want this. (450 extractor with a 458 case)  |
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