Can you put R30 in a 2x4 wall?

You cannot properly put a standard R30 fiberglass batt into a 2x4 wall and achieve R30 performance because a 2x4 wall cavity is only 3.5 inches deep, while a standard R30 batt is about 8.25 to 9.5 inches thick and designed for a 2x8 or 2x10 cavity like an attic or floor. If you compress an R30 batt into a 3.5 inch cavity, its R-value drops dramatically due to compression and you will actually get about R-12 to R-13, not R-30, and you may bow the drywall and create air pockets. For a 2x4 wall, the maximum batt that fits properly is R-13 to R-15, with R-15 being a high-density batt designed for 3.5 inch walls. If you want higher than R-15 in a 2x4 wall, you need to use closed-cell spray foam at R-6.5 per inch which can give you R-22 in 3.5 inches, or add continuous exterior rigid foam like 1 to 2 inches of polyiso over the sheathing, which adds R-6 to R-13 outside the studs and also reduces thermal bridging. So the answer is physically you can stuff it in, but thermally you will not get R30 and it violates manufacturer installation requirements and code intent.